I have just read yet another story where a female character is said to have either A, B, C, DD, etc., breasts-and, yes, I admit it, I've done it myself in my own stories! But just to clear up this common misconception, breasts do not come in these sizes; the cups of the bras that their owners wear do, however.
Cup sizes are a relative term which express the relationship between the volume of the breast tissue that a person has-which incidentally can and often does differ in each breast-and the garment that they need to contain said tissue correctly and comfortably. This sizing can vary between a bra's manufacturer and even its country of origin, for example. If any male gets the opportunity to do so, ask a female that you know well enough how easy or difficult it is to find a bra that does its job properly.
Of course a person can look at a woman and make an estimation of her vital statistics, including the size of their breasts, but rarely will this be as either as accurate or as clear cut as most fictional accounts suggest.