I wear the same size shorts in every brand I have tried. Right now I only wear Terry Bellas and Shebeest S Pros, but I also have Sugoi and Zoic liner shorts that came with more casual cycling wear. They all fit me fine.
I have never worn PI because they don't make x-smalls. Or, if they do, no one sells them.
I may be in the market for more shorts as a couple of my Bellas are wearing out. My first inclination is to stick with what I have, but I've been having some issues lately, and it may be the shorts. I don't know. I have my saddle tweaked finally. I don't want to waste money on things that hurt when I ride!
I used to have a pair of Shebeest shorts (they were called Double D?, something D) that I wore on the one century I did. The chamois was a bit thicker than what I usually like, but they stopped making them. I also had some triple S Shebeests, but I found the chamois didn't come up high enough int he front.
Now jerseys, pretty much the same. I always where an x small, except for Shebeest, which I can't wear at all. They are way too big and long. Oh and LG. All of their cycling clothes are made for women at least 6 feet tall.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport