I just bought this 4.3" touch screen:
I also bought the ARM board they suggest that you use with it.
I am going to be making a double DIN sized enclosure for my Honda Element. I am using a small LCD because I intend to have mounting places for an iPhone right next to the display.
Note that the LCD is a bit too tall to mount in portrait orientation in a double DIN. I will probably make a custom housing that is the size of my car radio - about 1cm larger than double DIN, just enough for it to fit.