Here is my 8.5' by 16' theater:
It started as an unfinished attic area on the second floor without walls, ceiling, electrical or HVAC. As I posted in my build thread, I found this site way too late into the build (essentially when all of the room basics were finished).
Although I would have done many things different, I'm still overjoyed every time I go into it to watch a movie.
The only things left to do are a riser for the back seats, art on the walls (framed movie posters), a sub replacement and a player upgrade to OPPO.