Once you've decided how big a front speaker you can fit in, you can look at rear speakers and make a decision on them. You should not, in theory, require any rear speakers, since there were never any rear microphones in the recording booth - it is stereo, remember. The addition of rear speakers on every car audio system except a DVD based Multimedia one is technically not right. However, since the rear walls of a concert hall or recording studio do create some reflections of sound (the concert hall a great deal more obviously) the rears are often used to reproduce these reflections. This has nothing whatsoever to do with whacking a set of 6x9 drivers into the parcel shelf and turning up the gain on your 1600W RMS amplifier! Since there are no dedicated rear left and rear right channels in a stereo world (don't confuse the fact that even a source unit that claims to have "front and rear RCA preouts" in reality only has 2 sets of LEFT AND RIGHT pre outs with their levels controlled by a fader) you need to be aware that if there is a gentle female vocal on the extreme left of the soundstage way deep in front of the ensemble, the left hand rear speaker will also try and play this which will somewhat spoil the effect (read "completely knacker it"!)
