just like everything else there are many variables to take into consideration. Start by reading the stick about seating distance and size. Then there are your mains.. how much room do you need on either side.. if you can not fit the size you want (taking seating distance into account) will building a false wall with acoustic transparent screen material be a better fit? or will seating distance be to close for AT and you will be able to see the weave.. What color are the walls? do you need gain? etc.. After reading the sticky and guessing at the largest size you think will work, it is always a good idea to get the projector first and experiment on the wall watching from the seating location. You do not have to mount the projector for this test, just put it on a table or chair..