What does it mean when a guy asks you what size bra you wear?

I don't know much about what happened there with you. But let's see if i can help you. if you know that guy very well and you both have spent a long time together, i mean u know each other very well then just take it as a joke and he is trying to do fun with you. but if you don't know him then he is flirting with you or he had bet with his friends to dare to say you like this. or he wants to check out that whether u r like bad girls or good girls. I hope i helped you. Or if not then i really apologize.