Their compression sports bra did basically nothing for me, let alone get me flat. (I'm an F cup, for reference.)
If you look under the "specifications" tab on their product pages, it gives you a sizing chart. The sports bra appears to go by the band size, measured as you would for a regular bra.
While the sports bra wasn't worth it for me, if you go for one of "extreme" options that actually compress, like the tri-top, I've found Underworks binders flatten significantly better and last a bit longer than GC2B. They also provide more coverage, meaning you don't get that pesky problem with your nipples slipping out of the armholes in public. The fabric is indeed less comfortable, but it's more than made up for by the whole not falling out in public thing.