If you're 11, you still have time to grow so don't worry about that now. Plus not many girls your age even have actual bra sizes.
But if they end up staying small, so what? Anyone who makes fun of you for that isn't a worthy person. Because seriously, what kind of respectable person would judge others based on the size of their bra? Only people who are jerks or really shallow would do that.
So please don't stress out about it. You are still young and there is more to being a girl than just the size of her bra.