What is the average size of projector screens?

This would mostly depend on how strong your projector is. The local drive in theater screens are 3 stories tall, but their projectors are water cooled. More importantly is the ratio of the screen to the size of the room and the position of the viewer. My home theatre is 14' wide by 22' deep and I have a screen that is about 10'diagonal (in a 16:9 ratio).