check out this thread:
smallest touch screen size?
not very helpful though really. you could always consider an iphone/ipod touch install, all though that'd be a lot smaller than 6". Alpine does a non-pc compatable 6.5" screen.
although it's not a touchscreen, i quickly found an english site: http://www.flyonthewall.uk.com/6-inc...74a5b8350f9577
that's a 6" inch monitor, however it's av video, so you'd have to run an adaptor.
really this kind of question is something you could answer with a few google searches, but it's to the point of hypocritical for me to even mention that due to my post count.