It's a 1.5 din stock. The problem that I have found with GPS in the head unit is it does not update fast enough when you go off course. I think Shane hit the nail on the head and it's probably better to just have a seperate GPS unit and attach it to the suchen cup on the windshield when you need to use it and just get a good head unit.
I too would love to find one that does both well but there isn't much out there unless you go to a full size DIN. Eclipse do seem to be the better reciever/GPS units but cost quite a bit and won't fit ur stock location unless you have it custom made from fiberglass.