Nick - you might want to look at seating first before committing to a room width. At 12 - 13' wide, you will be pretty much limited to three chairs across the room and still have enough room to walk around them. If I ever build another HT, I plan to build it 16' wide so that I can have four seats. The length will be around 24' so that I can have a second row of seats and enough distance from the seating to the SB speakers for optimum sound for those channels. 24' depth will also provide enough distance for wide channels or a second set of surround speakers, if I decide to go that route.
