Depends - if you want to use one of the walls, the maximum screen size would be 11 feet wide and 9 feet high (duh!).
Unfortunately, that doesn't match the aspect ratio of any of the commen TV or movie formats, so you'll have to decide how you want to match the picture aspect ratio to your wall. In addition, it will be problematic to sit (or stand) in this room without occluding part of the picture.
Oh, and you'll need to check for a projector that can do the 11 feet wide image at 10 or less feet projection distance.
For example this one :-) :
http://www.sony.net/Products/4k-ultra-short-throw/