Room size

My room is remarkably similar in size as yours. I consider it a major compromise in that it forces me to listen nearfield with the system under the higher ceiling height. My room is approximately 13X26 with a ceiling sloping from 14' down to 8'. I'd consider it a medium sized room. System is on the long wall. I've used both monitors and floorstanders in this room. Both can work very well, but I tend to prefer the fuller sound of floorstanders (currently using Coincident Super Eclipse III's). You need to consider the volume of the space as well, and I think this is why the larger speakers work better for me. I wouldn't go much bigger than a small to medium floorstander though. I also would be prepared to add some treatment based upon the actual room, contents, and system.

Onhwy61 made very good points. Kind of what I was getting at in the end of my post, but better stated and more thorough. I agree, it will depend largely upon the room, construction, contents, treatments, system location, room openings, etc. Unlike him, though, I have not heard larger speakers sound up to their potential in a smaller room, and have much more frequently heard problems with that kind of arrangement. That's not to say he's wrong and that it cannot be done, it simply has not been my experience. More often than not I've found it visually and acoustically overwhelming with a distortion of scale, in relation to the room, that tends to inhibit my suspension of disbelief. I've also experienced these kind of setups can often limit soundstaging potential. Notable exception have been asymmetrical corner arrangements. Again, this is in my limited experience and YMMV.

I use a sub as well, but tend to turn it off more for music as I think, in my case the single sub is slightly detrimental to imaging, and my speakers already go down pretty low. Two or more would be better, IMO.