Neither have I. I guess I'd buy one medium because it would protect the majority of the area if it truely wasn't big enough...unless of course I was handy at using a sewing machine and could alter the saddle. Too big, and I'd think it would not stay on the back where you want it.
Do you have a seller In mind that you could ask? I might ask about how the straps adjust.
Hopefully someone with more experience will help out.
CG