I do everything in oxygen free, 12AWG. That way I don't have to think about it as the price difference between gauges is nominal for a 100' or 250' spool. FYI, any in wall wire needs to be CL2 rated.
I might not need 12 AWG, but it is so inexpensive and gives me peace of mind.
Yeah, what beezar posted above as again, with 12AWG, you don't have to think about it.
As to the room's acoustics, to get an idea of how sonically active it's going be, once furnished and appointed, put on some hard leather shoes and toe tap your way around the room, listening for echos as you lightly tap with the leather'd sole of the front of the shoe.
Tap, echo, tap, echo, tap, echo, t.....
Good luck with that one.