I have the same projector in my theater which is 16'x20'. I am using a 107" x 60" Stewart Studiotech 1.3 gain screen. The picture looks fantastic IMO.
I have brightness at 61 and contrast at 55 (used to set it at 60 but cut it back to improve detail in bright scenes). The signal is coming from a Toshiba SD6200 Proscan DVD player through an Extron Transcoder to the projector. From my viewing distance of about 16.5' (front of reclining seats are 14' from screen) scan lines are not apparent with anamorphic dvds. They are slightly obvious with non-anam. dvd's in bright scenes, but that is no big deal since most stuff today is anam. I use to have a 2.0 gain screen but didn't like the hot spotting and color shift with some scenes. Believe me, you don't need a 2.0 gain screen. 1.3 is plenty and it produces a much better picture. The projector puts out plenty of light.
You'll find that on a screen this size, some dvd's may look soft. That however is the result of the DVD quality and not the screne or projector. Check out Dinosaur or Red Planet and you'll see just how good your system is. You will love what you see if the projector is set up properly.