Hmm, I would run as big as a display as possible.
I've run a Lilliput 7in touchscreen (yes... cheapest of the cheap) and there's no way I can/would use it in an automotive application, especially to tune.
I want to do the same with one of my cars eventually, i've got a few mini-itx motherboards and lots of laptop parts to make a very small PC. I'm also thinking about rewiring or hacking a small external numpad and trackball mouse to put in the center console to facilitate data entry and MP3/GPS usage.
Good luck, can't wait to see some progress!
Mario