Yeah, me too! I have so much trouble trying things on in stores, they always seem ok until I get them home.

Height 5'6" weight ~125 lbs. I tried to measure around the widest part of my backside, standing up, without squashing, kind of tricky, and it's about 36".

I am usually a size 6 for Columbia shorts, M for Sporthill xc pants, have Terry club shorts in M and they are fine. Hope that helps.