I used to have a 12c. You can see it on David Van Edwards' site. I bought it second hand, so had no choice in model, woods, measurements etc. My understanding is that the larger instruments work better. I didn't have it long enough to get into the specific repertoire for it, and to be honest, the repertoire is not as great as that for the 11c or 13c, and it felt like neither one. But it needed someone to specialise in it to bring out the riches.
Rob -- To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html