Gps geek help needed

As I'm doing a project car that I hope to use on long hauls and I will be using a hidden radio I'm wanting to make a plate to cover the existing radio cutouts and put a GPS system of some sort there. It's the plain jane dash with no junk plastic. I will be making the plate as wide as the heater control bezel about 7" and about 3" high. As I am totally stupid when it comes to technology I NEED HELP.
What can I buy that would fit into that space without sticking out don't need anything fancy. Please help me out so I do not have to spend hours researching something I know nothing about