I don't have very large moobs, but my ribcage is a decent size. My ribcage below the moobs is 40 inches and my moobs are like 42.5 inches. So I got an extra large which is 41-44inches. It slides on pretty easy overhead. Also it doesn't take much pressure to flatten them, I don't have much breast tissue and the little I have isn't dense at all. Where the binder ends the measurement is like 38 in, so the binder doesn't put hardly any pressure below my moobs. It puts slight pressure and my breathing is a little restricted, but I could go to the gym and work out without any problems.
I decided to go with a x-large cause 2 years ago I got a medium cause someone told me to go down a size, then my rib cage was 38 in. It was near impossible to get on, and I had epic fat spillage from the arm holes. I dont have much if any spillage now, but the arm holes are small, I really wish the armholes were bigger.