Determine the optimum screen size based on room dimensions, audience size, seating arrangement and the capabilities of your projector. A common formula is that the screen height should be approximately equal to one-sixth the distance from the screen to the last row of seats. Ideally, the first row of seats should be approximately two screen heights away. The reality is that you don't want the projected image to be so large that it becomes too dim. Smaller images are brighter and sharper.
Larger images have bigger impact.
Find a happy middleground that works for you. Use a white wall or hang a white sheet. Wait until nightfall, put your projector on your car hood and project your film onto a white garage door.