Placing your seat 1.5-2X screen widths works well for scaled DVDs. Personally, I'd try for something in the 6' wide (80" 16:9 diagonal) range.
I'm currently running a 6' wide screen with seating 12 feet back, and find it a bit small at more than 11 feet and a bit soft at 9 feet so I'll end up with a 7' 16:9 screen when I move the projector to the ceiling.
With 4:3 source coming from DBS, VHS, etc. the smaller 4:3 image is a positive feature.