well, I thought and thought about this but, I think I will build it 15.5'w x 19'L x 7.5'H

if I space the subs across the front ( hopefully this will sound best ) but at 1/3 intervals. I would hav then 5ft from the walls and 5ft apart. this will put the M80's closer to the walls and about 2ft away. giving me 11ft seperation. I am sitting 11ft from the front screen which will be 102".and allows for 1ft spacing between the sides of the screen and the M80's.

Pioneer sc-1525
M80 HG Cherry