You could try a Joye306. It was my starter model. It's shorter than the 510s and a tiny bit thinner. The 306s are fully compatible with the 510s -- the batteries and atomizers are totally interchangeable and the 306 batt even fits the 510 PCC, so you only need to buy a battery and atomizer to try it out.
The 306 batteries are automatic-only and a bit shorter than the 510 (of course, shorter batt life too), and the atomizer/cartridge is shorter because there's no mouthpiece sticking out.
(It's an exposed atomizer - you pull the cartridge off and the atomizer is just a short metal tube with the bridge and wick sticking out. You can dip it in a contact lens case rather than dripping on it.)
It's available at Electronicstix.com and ecigsupply.com (among others, I believe).