That's what window plugs are for

I've got a large one I'm fitting a plug over in my theater (WIP, see sig). If it's light controlled (plug if you care about sound, heavy shade if not), place speakers in front and build out your false screen wall (
http://www.avsforum.com/forum/19-de...n/837848-minimalist-approach-screen-wall.html) in front, then your entrance to the room will be from that doorway in the rear.
A 15' distance will work though and you can still get a plenty large screen, just need to consider that you can't bump your seats all the way back and still do 7.1 surround (you can do 5.1; 5.1 surround is on the
side, not the rear). Your viewing distance becomes the limiting factor, not really the projector.
For instance, a BenQ 1075 can do a 140" screen from like 13ft (
http://www.projectorcentral.com/BenQ-HT1075-projection-calculator-pro.htm), but you'll likely have to do a bit smaller to accomodate your central speaker.
EDIT: Also, a good chunk depends on whether you actually want a formal theater lol. I'm thinking rows of seating, but if you're thinking sectional, then the wider approach will work well too.