Are you going with a projector/screen setup? If so I would go as large as possible, so you can have some good seperation of all your speakers, especially the surrounds. Also, when building he room, try no to have the length, width, and height divisable easily by each other.
The worst room for peaks and nulls in frequency response is one that the dimensions easily divide into each other.
for example:
A room 15ft x 26ft x 9ft would be good.
A room 16ft x 24ft x 8ft is not ideal as 8 goes into the other dimensions evenly.
A room 24ft x 12ft x 12ft would be even worse.
