You may try Google-ing "screen size calculator". Many recommend a factor of 0.67 to 0.84 seating distance to screen size ratio. Example: 11ft seating distance; 11ft = 132inches; 132" x 0.76 = 100" screen size. We recently finished our basement home theater and have a 100" screen with a seating distance of around 11ft. It seems just right, no head sweeping to see the whole scene. It looks great in both 4k and HD. BTW, I can't say enough about the Viewsonic PX-727-4k projector. Good luck!