[Always *]

I am sorry your parents are taking this approach. I could understand if they just wanted you to get clothes that fit instead of clothes that are too big or too small, but the strange focus on a certain size sounds pretty strange. Even people who get clothes that fit perfect cannot buy things in a certain size all the time (eg. I'll wear a one size in one store but need the next size up at another store) because not all clothes are designed the same, some brands run smaller or maybe they just don't have the space allowed for big boobs in your "typical size" etc.

Have you tried to talk to your parents at all? Have they at all tried to explain this focus of theirs? Perhaps you could tell them that you need to be allowed to try on clothes and buy the one that fits best or something. I dunno...