I got an XL compression bra. I am usually a large. I will say i got info on compression bras from breast cancer.org. After surgery there is a brand called DALE the hospital gives. I ended up with fluid behind my right breast that need an aspiration twice. I was told I would need to have drains back in if I do not compress properly. Yikes. I bought extra large sports bras and stuffed the inside right side. That solved my fluid pocket having fluid. I am actually a large and bought those first. Too tight!
Below are bra makers for breast cancer patients. I heard good things about fit, but I went and bought a couple good sports bra myself. I would say a small may be more comfortable. It does not hurt to read up on bras and how to ensure fit.

There are also many other brands that carry bras and active wear for women who’ve had surgery for breast cancer. brands: Athleta, Amoena, AnaOno, Anita, Coobie, and LuisaLuisa.

This information is provided by Breastcancer.org.