I just bought an Epson EX51 video projector. It is mounted in a room where there's a distance of 12 feet to the screen. I have an 80" 4.3 aspect ratio projection screen, but the smallest image size that the projector displays is a 100" image which causes the image to display beyond the 80" screen. How do I decrease the image from the projector so it doesn't go beyond the 80" with the projector mounted 12 feet back?

If no adjustments possible, move projector closer.

With that projector, you will have to move closer. You'll have to be between 8 feet and 9 feet, 7 inches. You can accomplish this by either moving the projector or the screen. Once you have the projector within that range, then you can adjust the zoom to fine tune the size.