I think you could fit a smaller plasma display (like 36" or so) on hinges or a track that could come down from the roof.
It wouldn't be practical for driving, as it would probably block the driver's view. Also the close proximity to the screen would be pretty friggin difficult to use.
Dude, it's your ride. You need to figure out what you want. The commonly-used screens have measurements posted here. For anything larger, go to the CompUSA with a tape measure and make some notes.
Oh yeah, don't forget to measure your dash.