Im thinking about putting a large screen in my trunk for show purposes
Largest I could find is 18 inch though; want something big. Big and widescreen would be ideal

Where did you look? 21" is a pretty standard PC LCD monitor size these days. Dell has a well priced 24" widescreen LCD monitor. But it you want bigger you'll probably need to look at LCD or plasma TVs.

I think 17 will look ridiculous even on a bus!

but if thats what you want fine by me! its your money anyways!

If you have readbility problem with a 7 or 8in I understand, but have you given any thought to 10,12 or even 15inchers? 17 is huge even on a desktop....