There are several factors to consider when selecting a screen size.
1.) Brightness.
Most folks don't go over 52" x 92" , or 106" Diagonal with a 1272 projector. Then there are those who say that anything over 80" wide is pushing it. 92" wide or smaller, you can use a 1.3 gain screen. Over 92" wide, you'll need a screen with a gain of over 2.0.
2.) Distance from the screen, scan lines, and perceived sharpness.
If you use the projector with a line doubler, like the iScan, you'll see scan lines from 15' away on a 92" wide screen. With a better processor, you can eliminate the scan lines and sit closer to the screen.
So, based on your room size, (assuming your going to have two rows of seating), and your projector, you may want to consider screens in the 80" to 92" wide size range.