Not taking anything away from the guys/gals with mirror backup displays, but if you like bigger, the nav radio uses the complete screen for display (if you have the nav radio installed).
My wife needs gps, but it's not in her car, I have looked at a couple of gps/reverse camera units. The camera attaches to the rear and talks to the portable gps wirelessly (is that a word?). You get the best of both, bigger backup screen, portable gps.