Agree with Chick, the shank length you need is determined by how close your heel falls to their side when you let your leg hang neutral.
If your heel is really far off the horse where you have to move your leg a lot to make contact, then you'll need a longer shank. If it's close and your heels are nearly touching the horse's sides with a neutral leg, then you want shanks as short as you can get them to avoid accidentally sticking him with one.
You might want to browse around this site, they've got a few dozen different types/styles of spurs and lots of them have super short shanks. Spurs & Spur Straps - NRSworld.com
Oh, and another thing. I don't know if you're aware or not, but you'll want to make sure and buy a set in "womens" size. Men's spurs will likely be far too big and they're a pain to try to bend them small enough to stay on your foot.