I have a 92" screen and sit 10' away. I would not want to be any closer than that for this size of screen.
Regarding pre-wiring, do yourself a favor and run a subwoofer cable (or a coax cable with an RCA termination) to several different points in the room. There is just no way to know where your sub(s) is/are going to sound best, and pre-wiring to at least 2-3 locations is going to save you a lot of heartache during setup.
Also, this positions you well for the possibility of two subs (either immediately or in the near future), which I think in your situation [huge room] could give you very noticeable improvement in sound. I'd do a sub run to each corner of the room at a minimum since you have the luxury of being able to do so now before the drywall is up.
Also be sure to run conduit (PVC pipe or corrugated tubing) from the main equipment rack location to your projector so that you can rerun cables in the future easily without tearing the walls apart.
Jason
