There is actually quite a bit of room behind our radios. You should have no probs with just about any aftermarket radio you choose. The stock radio is almost the same exact size of most aftermarket radios anyway. Yes, its fine to use the radios 4 speaker channels to drive 6 speakers. The front door and dash speakers are tied into eachother, but they have been wired to keep the total load on the radio the same as the rear ones, do dont worry there, i've been running with and without an amp to all 4 front speakers with only 2 radio channels for long times, and no probs at all. As for the size of the radio, just look for a DIN size radio, most aftermarket radios are this size.

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)