If you haven't already, see our page on sizing.
The simplest answer is: if you have just started your period, up through 18 years, the bud (teen) cup was designed for you. From 18-30, the blossom (small) cup will be a great fit. And for 30 and older, the bloom (large) cup is just for you. If you've carried a full term pregnancy, we suggest sizing up.
Feeling like you fall into more than one category? Let's say you're 25, and have carried a full pregnancy, but delivered via c-section and you are trying to decide between a small or large. Due to the nature of our product, and not being able to exchange it, is the very reason we offer a mixed pack. The good news is, the vagina is elastic, and there's a chance both would work for you, but if for some reason one doesn't, you'll still have one Bloody Buddy Cup good for the next 10 years.