I have 2 pair of PI shorts - the PRO In-R-Cool short and the lesser-model Select. The fit between the two is markedly different. I'd say the Select fits almost on the small side of a medium, and the PRO is on the large end of a medium.

I'm 5'6" 130lbs, for me the PRO is a little too big (I wear them anyway, but they're a tad baggy at the back & crotch, yesterday while starting up from a stop they snagged hold of the saddle & I had to stop & pull 'em up, thank God I was alone, on a deserted stretch of road!). The leg gripper of the In-R-Cool is ok now that my legs have gotten lean, but I can tell you this: if you have so much as a quarter ounce of fat on the part of your leg where the gripper goes, you'll have sausage thighs. I see Susan said they have fixed it, but then used a hot knife so it gets itchy! Good to know.

No complaints on the leg gripper of the Select. It also fits me perfectly, right now.

I like the front panel on the PRO a LOT, especially when hormones or unfortunate eating choices kick in. Wish the Select had it!

Comfort-wise, I find the chamois in the Select & PRO about the same, I don't notice a difference on long (50 mile) rides.