a 7" screen fits perfectly behind a sat nav surround but other than that a 4" or 5" should fit in the normal cubby I think fairly easily.
Seems a bit of aggro to go for just for a screen that you use very occasionally, I have the same camera attached to my HU, I would personally advise to save your pennies and get an aftermarket HU as it will look better and do both jobs
Maybe there are satnavs out there with an rca in for the screen