Don't worry too much about the size. V3.0 of my theater was about 15 feet deep by about 10 feet wide (I say about because it was a very irregular shaped room) and there was duct work and a support beam preventing a full 8 foot tall area. I had "way more speaker" than the room required, but liked the room. For the time it worked. I had a 104" 16:9 screen that fit nicely in the space as well.
Sure, now I am constructing a room with 8 foot ceilings that is 14 feet wide by 24 feet long, but that is just me getting lucky with the space...
2-M60s, VP180, 8-M3s, SVS 20-39PCi, DIY Sub, 8-Shakers, JVC RS45, Anthem MRX-1120