That depends on screen aspect ratio, projection artifacts, projector light output, what trade offs you prefer in the video (you might accept more visible digital artifacts to get immersion, or choose a larger + dimmer picture), how much room you want to give the left+right speakers, how many rows of seating you want to accomodate, how much you want to compromise front-row performance to get a better picture elsewhere, etc.
The best answer here is to buy your projector, shine it on a white wall, watch some movies, and figure out what subtended field of vision works best for you. Then use the largest screen which allows the same subtended field of vision without causing brightness problems, leaving insufficient room for your second row, crowding the speakers too much, etc.
IOW, if you find that you like to sit about 12' from an 8' wide 16x9 screen (this is average) you could sit 15' from a 10' wide screen or 30' from a 20' wide screen. This can also be worked in relation to image height, which seems to relate more closely with perceived size than width. All else equal, once the lights were off you'd get the same experience from any of those combinations although the people sitting behind you would have a more immersive experience on the larger screens.
In the absence of digital artifacts and/or limited brightness, I'd try a 10' wide 2.35:1 screen with the front row 12' away.
