All about sizes

I am not a tall person and not especially large or small, I am more of an overall petite. This makes my shopping very complicated. I can look for only XSs and Os, and even those are often big on me. Stores like Target and other "fast-food fashion" kind of stores in USA have exceptionally large sizes. Their XSs are more like a M in a brand name store. On the other side, high fashion brands like Prada rarely cary 0s, they have 2s that are fit for a taller woman. I found that Bebe, A&F and Guess have the smallest sizes, with A&F carrying OOs and Bebe having a nice selection of XXSs. Finding the right size is difficult, which makes online shopping that much more difficult. Recently I bought Alexander Wang XS sweat pants on Net-A-Porte, I ended up giving them to my mom, who is a size 8.