Using THX standards you really want a screen that is about .66 times the seating distance in width. So, if you are seated at 12 feet, then a screen about 8 feet wide, or 96" (ish) which is about a 110" diagonal. If you like sitting in the middle of a movie theater, this is, more or less, the effect you will get from a screen that size. If you like sitting futher back, go smaller, if you like sitting closer, go bigger.
The projectors should both be able to handle the size, and then the ambient light issue with several large windows in the room are of CRITICAL importance. If you aren't controlling the light 100% then you really need to come down in screen size to compensate some. There are many factors, but really, seating distance, combined with your typical movie theater preference is key.