[@DarkwolfX37] Ah. I won't speak for anyoneone else, but in my own experience there are only three reasons that you would need to know specific measurements for your character. 

1. That information is of particular importance to your character (to use your example, a small breasted girl who tends to suffer from size jealously)

2. They are clothes shopping and that information would come up in conversation

3. You're writing an explicit sex scene ( and even then, this one is a big if)

Otherwise, it's just numbers that you most likely won't use and others would be better served with approximations (large, small, average).