ROOM SIZE QUESTION

You didn't quite mention what gear you were considering using in your room..this has a lot to do with what's workable/not workable in a given room..especially smaller rooms like the one your considering at 12.5X18! ]
My vast experience with dealing with all sorts of rooms sized, shapes, etc, is that, yes, your larger rooms(in general) tend to be easier to work with from a frequency response and balance stand point, and gives you more options for different type of gear/seating/speaker placement scenarios for best performance! Small rooms tend to be even more difficult to set up any given speaker/seating arrangement, to yield flat response from the speakers, especially if you need more than two seating/listening possitions. And make no mistake about it, you will have a tough challenge getting good balanced speaker response in that acoustically relatively small environment of 12.5X18X10!!! I've done enough high end custom theater/listening rooms of that size range to know better. If you end up wanting to push your couch or chairs towards the back of the room,so you can get further away from a big screen for example, that makes things even more challenging sonically!
I would suggest, that you consider either a one chair or two chair set up in a room of that size, as the primary listening spots, and try to balance things around that(those) chairs! Trying to get more chairs in there will be difficult to get great sound from all of them(although you can have some additional seating possibly with some sacrifice in the sound ultimately). If your mostly a "solo" watcher/listener, I think, with a little knowledge, experience, OR KNOWLEDGABLE HELP/ASSISTANCE, you could possibly get reasonably flat frequency response from all the speakers in a room of 12.5X18X10, yes. Then treating room reflections, and balancing out reverb and overall all balance in the room will take some more effort.
You didn't mention if the room was a "closed in" room, or "open to other rooms and openings" type room. This makes a great deal of difference acoustically!..so be aware of this. Infact, as a designer and acoustical engineer myself, I'd want to see that actual layout of the room first!
Your first priority in a room of that size, if music(I suppose 2 channel music mostly, yes?) is your priority especially, should be, one,getting ALL THE SPEAKERS BALANCED for even and equal frequency response(mainly bass in considered here!), and integrating the woofer(s) well also!!!! If you can't at least get your main 2 music speakers for your listening set up well, then you can forget really fantastic sound anyway!..it's just not going to happen. I would recommend setting up ONE SPEAKER AT A TIME(in mono/summed stereo)and get each one right from the main listening possition(s)! Start with left and right(this will take some learning/trial and error), then go to sub!(yes, sub!...so you can get the sub to integrated properly with the left and right and your seating possition!). Then, once you have that combo going perfectly, you can play around with the center channel to balance with the left/right/sub combo!. Then, do rears to balance with the sub possition, and for flat frequency response as well!
If you can pull that balancing act off, you'll be doing EXTERMELY WELL SONICALLY!..that supposing you have nice sounding gear in the first place(gear 40-50%/room and setup 40-50%/tweeks/5-10% roughly). Most people, again, CANNOT GET THE SET UP DONE WELL!...so the results suffer badly! Trust me, I've been around 1000's of setups over the years...thousands!
Placing speaker in a room, getting the seating possitions right, and playing with acoustics, is all science, skill, patience, and experience! learn how to do this well, and you can virtually work with any system and room!...within reason of course.
Now in regards to that 12.5 X 18 X 10 foot room...I see one potential problem, in that you may experience(if you do no mod's to the dimmensions)some frequency response build up at certain bass modes, causing a boomy sound at certain frequencies. This usually makes speaker and setting placement options difficult. It's difficult to see exactly all of what's going on hower, since I'm not seeing the actual room shape. Also, STAY AWAY FROM THE BACK WALL WITH YOUR CHAIR(S) ALSO! This will make getting anything except boomy peaky sound all but extremely difficult to get rid of! Try putting your chair in listening spots of either 1/5's or 1/3's from back to front, or left to right! If it's mostly a "solo/one-chair" listening room, just for your, then you can consider placing your chair in the middle of the left/right wall, and place the chair in either 1/3rd length possition, or 1/5th lenght possition from back to front. Then from there, you can play with speakers balancing from that seating possition! If you do two seats, then keep the two seats symetrical, and they will both sound the same, as long as room is symetrical).
After you can get the speaker system in good foundational positions from your seating spots, you can concentrate on treating reflections and echoes, reverb, whatever, etc.
Another thing to consider aslo is your type of viewing monitor/projector! If you use a big screen RPTV, you must consider that you will not be able to sit so close to it!(although you can sit closer than you used to with newer sets and higher def material) This often forces you to sit back further in the room. and the bigger the screen the more it will put you back towards the back wall, causing acoustic challenges, So be careful.
Even after proper monitor calibration, you still can't get as close to the screen, and as big of a picture with a rear projection big screen as you can with an overheard/front projector! If you go the way of a front projector(STRONGLY RECOMMENDED), you can get a much bigger, and better overall picture, wich you can sit closer to!!!...giving a much better home theater experience really. You can also get teh screen flush on the front wall, makeing for even more room for speakers, gear and seats!...another thought.
Incase you weren't awarer, rhe reason, even witih the same resolution sources, that you must sit further away from a big screen than good front projector set-up, is that the lenticular screen on a RPTV is too vissable up close, and you see the grid too easliy. In order to not be distracted by this, it forces you to sit back further..this is not a problem with overhead/front projectors!. In addition, with the front projector,the wow factor is just much greater with the larger picture, even though you mentioned music as a priority.(you'd be blown away with a good projetor and screen combo!!!!...it's really good potentiall...it's a thought)
For that room your sonsidering, for example, I would not recommend anything above 50" 16X9(max) HD for considering in RPTV bigscreen! You'll otherwise be forced against the back wall if you want a great picture.
Anyway, my suggestion, if you really want to do this right, and not make it an on going hobby, is to pay someone who's experienced to either do it for you, or at least help you!...whether it be via email/fax/phone, or in person!!!
There's simply SO SO SOOOOO much to consider in making systems sound an look great, in any given set-up, with any given room, with any given gear! The list is long in considerations to adress, indeed! If it's simply something you want to tackle your self, you will do well to learn learn learn, experiement, experiment, etxperiment, and do lots of reading! Picking up back issues of Stereophile GHT magazine, and reading all of Russ Herschelmanns "home Theater Architect" couldn't hurt either. I also strongly recommend F. Alton Everests Master Hand book of Acoustics as well.
Still, there's no replacment for experience. If you just wanted it right from the beggining, so you could relax and just enjoy your system, I highly suggest seaking professional assistance.
Anyway, it's all good...it's just that, with experience, knowledge, and some skill, it's WAY better! When you consider all the variables, and speaker/gear choices, and settings/calibrations, and proper set up condiderations, acoustics,and a virutally endless mirriad of other considerations of importance, etc., getting world class results is a daunting task for the average "newbbie" to the game!...just the way it is. Heck, even seasoned audiophiles often don't do it so well. Infact, that's the norm mostly!!
Have fun.
Audioalexander

Lthkeepr- WHEW! Thanks for the treatise! To your first question- the stereo system is Audible Illusions M3A pre-amp, Counterpoint SA-20 amp, Maggie 12 speakers and a powered sub. The room has two exterior walls and two interior walls, completely enclosed, and is on a cement slab foundation with carpeting. For the HT part, I'm planning on staying in "stereo" (not surround) for at least the near future and utilzing the stereo for sound. I've already considered a projector as a pull-down screen can "disappear" when not in use.

Music is mainly solo or with one other, HT would be with 2 to 4 people with an occasional couple more kids on the floor.

Think I can get this project off the ground or is it wishful thinking?

Thanks
Jim

I have a room that size and consider it acceptable for 2 people listening watching. I have a 60" tv (pioneer elite 610HD) and that's about all there is room for. Just consider your real plans for usage. I couldn't do anything else, but am wishing I had about 50% more room in length and width.

Well certainly you'll have an easier time setting up two speakers than 6+ speakers in that room! Although maggies can be challenging to get sounding good in a room, due largely to the bi-pole nature of the speaker, and phase cancellation and comb-filtering, etc.
Maggies, ey? I am only a bit familir with the Maggie models, and mostly the 1.6's, 3.6's, etc. My exerience with Maggies is that the definitely make better, more practical, music speakers than HT speakers largely. They tend to be a bit delicate, and can be pushed to hard if not careful. Also, they are rather low sensitivity designs, with low impedance curves. This, for the record is the antithesis of what a good theater speaker is(mosly higher sensitivity, and efficient...thus very dyanamically effortless!) Also, movie soundtracks are several times more demanding than most of your likely music choices! Dynamic Dolby and DTS dig can play havock with systems, so be careful. DEFINITELY cross the speakers over at 80hz, and let the sub do the bottom end.
Also, I don't see a surround sound pre/pro in your list of gear. If you don't plan on using one, and just using analog pre-out's of your DVD player(processing Dolb Dig in the player), you're even further degrading the dynamics and overall soundquality from movies! YOU NEED TO GET SOME SORT OF OUTBOARD PROCESSOR FOR your DD/DTS material! It otherwise never sounds as good just playing through your 2 channel analog set-up.
You sollution would be to "loop" a digital pre/pro through an Auxilary or tape loop on your Audible Illusions preamp, and just click on that input when you want to play movies. That way you can keep your 2 channel issolated.
Another consideration with the maggies is that they are largely "sit-in-between-the-speaker" monitors!...just like all stereo bipole monitors(unless you're talking about diffuse rear/side speakers, and that's another story.). If you sit off axis, or have other people over, consider the off axis sacrifice and phase cancellation effect, which makes any seating possition outside the speakers focus sound somewhat filtered, muted, and bland compared to on-axis seating. I've found this problem with Martin Logans and Apagee's, and other planner type speakers as well for multi seat applications.
An upside to using maggies
Another alternative, one with probably better overall effect, pressence, and dynamics, would be use some less ambitious(albeit better suited for the job) wall mountable monitors for your HT dubties, and keep the Maggies on the floor for you music dubties!?! This way you can do what ALL the Stereo Mag reviewers/writers do, and keep two separate systems, even though it's all in the same room/location....it's a consideration anyway. You could even go 5.1+, and use a clean sounding receiver instead, keeping your 2 channel separate. Again,it's a consideration.
Remember, movie soundtracks have different demands and
attributes. Movies are not high purity audiophile recordings, that are untainted! Just go to any top flight local cinema and listen! What's demanded, is speakers that are dynamic, have good focus, dialog intelligeble, and have good pressence and detail! Yes, having uncolored, clear, neutral, refined sound is always a plus, but it's not what's ultimately needed to maximize movie soundtracks. They're largely overdubed, mixed, layered, re-edited, over-scored, and processed recordings! You'll always have "boxy" dialog, and sonic flaws, and indistiguishable sound effects and sonics. Making trying to pressent movies in an audiophile light is not really that practical to a large extent. So, when you say your using your more delicated and refined gear for HT purposes, I just want to try to put things in perspective, and that's that your more laid back, mello, delicate, low sensitivity, rather limiited dynamics, audio system is much better suited to your music applications, if your going for "WOW FACTOR" for movies that is! Not to bash your gear, contraire! I'm rather trying to be honest and make you aware of what to expect.
If you wanted, for example, you could easily, and cheaply, get some more than adequate higher sensitivity satalite speakers and a receiver, to mount in the room, and you'd probably be way ahead of the game for movie listening!
Guess you gotta weigh your priorities. I know you mentioned "mainly music", so you can feel assured that at last that medium has a good chance of very good success as is.
Anyway good luck on your project, and have fun...it's all good.

Thanks, everyone, for your input. I elected to not get this particular home for a number of reasons, not the least of which was the room in question was not really adequate. To Lthkeepr's comments about the suitability of the equipment for HT, I realize that the current system is a music system and will probably go with a modest separate HT system in the future, but will need to utilize the current system for HT in the immediate future. For me, HT is just not at all a priority, but a nice bonus.

Thanks again to all.

Regards
Jim