search in
www.distrowatch.com
I think I found quite a few USb flash disc bootable distros there, one of them was 64MB ! I think it was called DammSmall Linux, the site also displays all the packages included, you can check for K3b.
I personally use Slax, it is 178MB, and fits on those small discs (200MB disc), it has KDE 3.2.3, and K3b