How do you know your bra is too small?
Test by pulling out the bra band at the back with your finger. If it fits right it should be easy to pull out 2 inches from the body. Just as in the previous example, go up one or more band sizes. For each band size you go up, you must go down a cup size.
How do I know if my bra fits properly?
You should be able to wear your bra on the loosest or middle setting when you first buy it. This allows you to tighten it when the band stretches over time. If the band is digging in and leaving marks then it is too small. If you can get more than 2 fingers under the band, it is too big.