Hahah! Kinda funny. But I understand what you mean, actually. The problem is is that it's NOT THE SAME, universally. Yes, that's right. The bra has two measurements - the number and the letter. The number is the number of inches around the woman's ribcage. The letter is SUPPOSED to be the size of the part that covers the actual breast, but on a woman who is very small around the ribcage, an A cup is much smaller than on a woman who is much large around the ribcage.
For example, when I take my measurements the official way that the girls in lingerie stores show you, it tells me I should be wearing a 46DDD. However, when I actually buy a bra that fits, it's a 40D. If you go to the store and pick up a bra that's a 34D (very small ribcage, larger boobs) the cup will be about the same size as the cup on a 40B, because the industry automatically assumes that if you're bigger so are your breasts. (Because hey, if she's fat she had better at least have a big rack!)
Really, there's no such thing as saying, an A cup is this size, because it changes from woman to woman. However, you can usually estimate easier if you think about her proportions. If she's a tiny girl with boobs that make her look like she'll fall over, she's probably a D or so - because her body is small enough that in comparison, they look bigger, whereas a taller or wider girl with the same size boobs, they look smaller.
A good rule of thumb is, if she's pretty much flat, she's an A. IF she's got just enough that you can tell they're boobs - assume they're Bs, because girls who are on the line between A and B always like it when people think they're closer to B. If it looks like it would about fill your hand without any spilling over, that's usually a C-ish; if you could only cup the bottom with your hand, they're closer to Ds. If you could just kind of stick your hand under the bottom and use it as a SHELF, they're DD or bigger.
Also remember that clothing and especially bras are deceiving. No woman is ever happy with her body, so girls with little **** wear pushups and shirts designed to make them look bigger, and girls with huge **** wear minimizing bras and tops to take attention away (sometimes).
It really is difficult to just look at boobs and know what size they are. I happen to be really good at it, being both female and a lesbian. Cos, well, HAVING boobs you learn a lot about them, and liking boobs you learn more. So. *cough*
Just remember, there's a lot more to a woman than the size of her boobs. And honestly, the most important thing when it comes to boobs is to remember - they are not an etch-a-sketch, please do not twist the nipples like knobs, kthx.