Here's a quick and dirty (yet healthy) way to determine bra cup size. This supposedly works well for men though I can't imagine why.
First, a small intermission.
1. Remove partner's bra.
2. Cup individual bosoms in each hand.
3. Use guide below to figure out cup size.
If bosoms are approximately the size of lemons, your partner is an A cup.
If bosoms are approximately the size of oranges, your partner is a B cup.
If bosoms are approximately the size of grapefruits, your partner is a C cup.
If bosoms are approximately the size of melons (they don't say which type), your partner is a D cup.
I'm guessing that if you squished a lime more into the shape of a bosom, you'd get a AA cup.
After typing this up, I googled the theory... and it is rather inaccurate. However, I approve of the pictures of brassieres with their cups laden with fruit for evidence-based principles.