I think people normally order the length for how tall they are. The Progressive HD shocks are heavy duty, so I would think even at 12" they will work properly. If at stock you sit and still have plenty of room get the longer shock. If you are short legged and would like it lower, get the shorter one. I am short legged but will not lower any of mine, I like to keep them where I can get the maximum lean out of them.
Hope this helps!
David