Hereditary Size?

Both my mom and my aunt have fairly large . Around D's or so. But my grandmother has smaller . I on the other hand don't have large s. I'm not sure if they're still growing, because I'm still at a fairly young age and we all grow at different paces. But my cousin is a D, and she is younger than me. I don't want large , but I also don't want small ones. What is the chance that my s will be the size of my mother/aunt? Or is it a possibility that I will have small like my grandmother?