Good question. When I want something loose and not overly obvious, I wear 5X or 6X. If I want something tight, I go for a 2 or 3X. But all tops must pass the stretch test. I literally hold them up to my tits in the store and pull on the fabric to see if it stretches enough to cover them. This gets me mighty funny looks, but it's better than having to return the tops later, all torn up and having them argue about their return policy. Although....that does make me feel like the Incredible Hulk, lol. 
Ha,ha that Hulk part reminds me of a Benny Hill sketch, where a woman turned into a rage, but instead of getting huge and muscular her breasts expanded and ripped her shirt....
As for arguing about the return policy, what about this:
You wait until a guy is behind the counter, not a woman. Then you tell him how ripped them, expanding your bosom to show it, wearing a little hiding, but "rip-proof" t shirt. And coo, bosom still expanded, how grateful you'de be to get your money back. He'll give it to you gladly. You know men....
