Listening Room Dimensions... I'm building one and need your help

Give Art Noxon at ASC a call. He'll work with you all the way through the build. I've been in a room constructed using his wall products (that of Audiogoner folkfreak), and it was incredible.

As an exercise consider a contemporary performance Hall and note the 
design features.
Expanding from front to rear and overhead dimensions are expansive.
Avoid parallel surfaces and plan ahead for treatment.

Early on in a project like this is when the most far-reaching decisions are made, and it is when your knowledge and experience are the least. So this is arguably when you will benefit the most from the services of a professional. I am a reasonably educated amateur (been building speakers for a variety of applications for over forty years), yet what a real professional acoustician can do leaves me in the dust.

I highly recommend Jeff Hedback of Hedback Designed Acoustics, he is a multi-award-winning studio designer and also does superb home audio room designs and acoustic treatments, with budget and decor very much in mind. Jeff has extensive expertise in what you are trying to accomplish at BOTH ends of your room, AND he is still affordable. Bring him in as early as you can in the process and it will be the best investment you could possibly make.

Just to give you a taste, here is an article Jeff wrote for GIK Acoustics. It does not delve into room design itself but that is likewise an area where he has world-class expertise:
https://www.gikacoustics.com/diffusion-by-jeff-hedback/

Duke

What Duke said.

Other than that, realize that no matter how good the room design a lot of that band rehearsal stuff you're talking about putting in there is gonna be nothing but bad for the stereo. The sound a drum makes is from the skin moving, which is supposed to be from being hit but trust me its gonna move just as well when excited by the air playing music on the stereo. So you are gonna hear that drum when playing music whether you want to or not. Which same goes for the cymbals, guitar, cowbell, all of it. Which is fine if you are fine with it. I sure would not be. Especially not if I spent big on professional acoustic design only to discover later the pro totally asleep at the wheel missing what some schlub on a website could see coming a mile away.

The ratios still apply regardless of how big you want to make the space.  Since this isn’t going to be dedicated, which you’re alluding to it being multipurpose, work the ratios the best you can based on what you want to do with it.  Most people who strive for a “great sounding room” rarely get it. They end up over-damped and dead sounding or there’s not enough damping and things are too reflective with more modern hard surfaces.  Odd walls and dimensions can help break things up and with furniture, rugs, carpet, wall hangings, there’s a lot that can help.  This is an area that can easily be overthought.  
https://www.stereophile.com/content/listening-room-forgotten-factor-page-2

https://audiophilereview.com/room-acoustics/ideal-room-size-dimensions-and-ratios-for-audiophile-listening.html

https://www.psaudio.com/pauls-posts/critical-room-dimensions/

Another "what @audiokinesis said" OR, if you do it alone, here are the ideal height - width / length relations to minimize modes. If H is for height and X & Y the two walls of a rectangular room, then H=1 
to X=1.9 to Y=1.4/1.3 is best
X=1.5, Y=2.1 / 2.2 is second best

The above are all good suggestions.  I did similar research when building my dedicated audio room. (You can check out my virtual system)  I must say, when I first listened to my room before any treatments were done, I recall a distinct disappointment relative to a small random audio room in my prior house.  

Just last night I was thinking how pleased I was with the transformation of this room.  The change was due to room treatments and tinkering with placement issues.  And I'm not quite done.  Still have some diffraction additions I will make.  

So ideal ratios are a good place to start, but like a fine musical instrument the room will need to be tuned for sure.

One reason for my suggestion of "hire a professional" BEFORE building the room is that there are differing schools of thought on the importance of room dimension ratios. I am quite familiar with golden ratios and their theoretical desirability, but in a particular application their use may not matter as much as maximizing room size, and/or they may not be practical for whatever reason... and if not, then what ratios do make the most sense?

Also, in a purpose-built dedicated audio room non-parallel walls are often appropriate, and if such is the case for a big dual-use room like this, might as well get those angles right to begin with.

Finally, construction techniques and materials choice make a significant difference. For instance, sound isolation to achieve a low noise floor will involve proper attention to the air conditioning duct work, as well as to any other openings to the room - including the wall sockets. And low-frequency damping behavior can be built into the walls for far less money than it would cost to add it after the room has been built. And so forth.

Duke

There are obviously many, many issues involved in constructing a good-sounding room, and room dimensions is only one. As long as you don’t design a room that’s nearly square then there are probably other factors to address that may be even more consequential to your ultimate result. The best advice I can give is to educate yourself on at least the basics and principles of constructing a good dedicated listening room. Knowing just the basics will put you in a MUCH better position to make critical decisions even if you end up working with an expert. I went through several books, etc. in trying to get a handle on this myself, and BY FAR the best thing I did was read Premium Home Theater: Design and Construction by Earl Geddes (no longer in print, but you can download it at gedlee.com). Don’t let the title fool you — this book is as relevant to building an audio-only room as it is to HT as the design principles are largely the same. More than any other source I found, this book explains both the theory and practice of designing and constructing a good-sounding room in a very approachable way so even a layman like myself can easily understand and comprehend them. And it also gives specific recommendations on the actual products to buy to complete a project, which I found most helpful. Before I read the book I didn’t even know what a resilient channel was, but now I’m confident I could instruct any contractor how to build a very good-sounding room (and I know nothing about construction). Even if I wasn’t building a listening room I’d find Earl’s book to be an interesting read. After all, I think most of us would agree that the room is one of — if not the — most influential components in our sound systems, so why not get a little smarter about it? Hope this helps, and best of luck with your project.

I agree with @bdp24  about the golden ratio though I don't think Cardas was its inventor. It goes back to BC time and is found a lot in nature. I'm going from memory now so ...You can google it

True @artemus_5, the ratio is not of Cardas' invention, merely his "re-discovery". He uses it in the construction of his cables too, the relative sizes of the various gauge strands of wire he combines.

@bdp24 

He uses it in the construction of his cables

I did not know that. I don't have any Cardas cables. Maybe I should try one?

My recommendation - don't do it. I simply would not try to make a premium two channel room that also would be for band rehearsal. I would find another place to rehearse. You may not have that option, but if you do, I advise you don't mash sonic attributes of the two together. 

If you go for the band rehearsal sized space, you will be building a cavern, not a room. The entire experience, including what gear you will need to run to make such a space seem appropriate will differ. Are you ready to pony up perhaps triple the money you have invested in audio to get the results appropriate for the space? 

It seems like you have lots of room to work with; why not build two rooms appropriate for their purpose? You can use a common wall, but use different construction techniques for each on other walls. You can send me the consulting fee if you do so. ;) 

For calculation of dimension based on Height online calculator.
Based upon creating as many resonances as possible.

http://www.mh-audio.nl/Acoustics/RoomDim.html