a 100" is plenty when you're seating at that distance or closer. If you plan things properly you can do a very cozy little cinema room in there. Mine is about 14' long by 9' wide and I've installed a 107" screen. Had to be retractable because of the room's door. Yours would be easier to do. The thing is, if you don't mind going for an 85" TV, as the previous poster suggested, then that would give you the best image without even bothering treating the whole room in black, changing lamps, (possible) projector noise, etc.