Try the AVS Forum @ http://www.avsforum.com/
What dimensions are you asking about? Room dimensions? projector throw? Seating position? Is your primary concern video performance or audio performance or both? Our company designs rooms both audio and HT for superior sound (but we also take into account the video performance). There are a number of very good books out there that can be used as a reference. One of the more comprehensive books, and not too technical is Alton Everest's Master Handbook of Acoustics 4th edition. Amazon sells it. Of course, we'd like you to use our services, but the book is a lot less expensive.