If you have no really specific limitations on seating, then you start with THX standards.
THX says that seating should be about 1.5x the width of the screen. If you put in a screen that is about 8.5 feet wide, about 10' or so diagonal, then you will want to sit about 13' from the screen.
The projector distance is ENTIRELY dependent on the projector you purchase and no two projectors are necessarily the same. Many LCD projectors have super zoom range which allows for a 120" diagonal to be thrown from about 12 to 24 feet from the screen. Yet, many DLP models have a very limited range and may only work 14-16 feet from the screen. (lens to screen)
Every piece needs a bit of research, and Projector Reviews, Projector Central, and of course Audioholics, can help you find some information on which projector and screen makes sense.
As an off the cuff recommendation: The Carada Criterion in Brilliant White is a great screen, and you should be pretty happy with the Epson 6500UB projector.