The general rule is that you want the screen to be .66 x the distance to your primary seating in width.
If you go to a movie theater and sit a bit back from center, then you can go a bit under this measurement, if you sit a bit closer in the theater, then you can go a bit larger.
I would recommend a 1080p projector.
I would also be very careful about your projector selection and screen size selection if you have already determined where the projector 'must' go. The wiring location should be determined by the projector distance, which is a factor of the zoom range of the projector, against the screen size, which is determined by seating distance.
Follow all that? It's about five factors to properly predetermine projector wiring location. Then you need to pull all the correct wires to that location, and it would be wise to pull conduit to that location as well.
If you are going with primary seating at about 12 feet, with a 1080p projector, and you like the middle of the theater, then you want about a 8 foot wide screen - 96 inches. That's about a 110" diagonal 16:9 screen.