I used a Lilliput 7" screen. It fits pretty good. Its a b!tch to mold in but. Food for thought, it's not huge but it's definitely not small either.
Also one more thing to think about, after setting a whole carputer up I think next time I will probably look for a midrange laptop with a broken screen, take the screen off and then make a tiny eclosure for that and mount up to the back seat. That way you dont have to go from DC to AC back to DC. THe problem enlies that most laptops are 19v so you would have make some circuitry to bump up the voltage but I would imagine with an EE or electronic junky it shouldn't be too bad.