It's not for everybody, I suppose, but I think 6x9 (cm) is a lovely size for contact prints. You have to get up close and personal with the print to appreciate it at that size, which to me makes it feel much more intimate and communicative, less like the one-sided conversation that is "art gallery" presentation.

That said, I think most labs can print at whatever size and proportions you ask for, though you may have to pay for the next largest standard size. If you want 6x9 inches, they'll probably charge you for an 8x10, and in fact they'll probably print it on 8x10 and trim to the requested size.