I'd definitely want a live sound room. Carpet is the worst thing ever and I couldn't imagine a studio application where it would be useful. (Maybe in the lobby). I'd definitely go hardwoord.
Shape and Size are where things get complicated. Assuming you could have any size or shape, I'd try to find that "Golden Rule" dimension. I can't remember the ratio off hand, but it's supposed to drastically reduce room modes. Acoustic guitars don't necessarily require a huge room, but I'd avoid anything too small or you'll be dealing with boxiness. When in doubt, the bigger the better.