I think you'd want to first decode the appropriate .vkb in /usr/share/keyboards/ to xml (this perl script should do the trick: http://idoru.metadreams.net/~kimju/s.../decode_vkb.pl), edit height/width values under keysizes, then track down a copy of gen_vkb to compile your xml back into a vkb, and finally load your new virtual keyboard.

Good luck, and don't blame me if you break something!

Oh, and if someone knows how I could add a ctrl key to the vkb I'd love to know how, it'd be nice to have one to the left of the spacebar