Guessing your cup size:
1) Reach up into your vagina during your period and find your cervix. (Ideally, do this a couple of times at different points in your period, because some women's cervixes move a lot during their cycles.) If you can easily reach your cervix without bearing down, the Diva will probably be too long for you. If you can't reach your cervix at all, get a cup with a long stem and don't cut off the stem before you try the cup.
2) See how many fingers you can fit in your vagina. If you can't fit three, get one of the smallest cups (small Lunette or Lady Cup). If you can fit three but it's uncomfortable, you might be able to handle a medium-width cup (small Diva, Miacup, Keeper, Mooncup UK) but shouldn't get a large, and you'll probably be better with a small cup if you have small fingers. If you can fit three fingers easily, a large cup might be workable.
What I'm basing this on:
I can reach my cervix fairly easily with my middle finger, and find the Diva slightly long--my cervix actually fits inside the cup, and the bottom is RIGHT at the entrance to my vagina even with the stem cut off. I can fit three fingers in my vagina but find it a bit of a stretch, and the Diva is mostly okay width-wise now (after a year) but felt too wide when I started using it (removal was painful).
I have short fingers with average width-to-height proportion. My ring size (right ring finger) is about 6.25 to 6.5 (M 1/2 to N in British sizes: see converter). I'm 25, no kids, a virgin but *ahem* otherwise physically experienced, used O.B. tampons in regular through super plus sizes from age 10 on. If I get a second cup at some point, it'll probably be the small Lunette--I have a feeling the shorter length and slightly narrower width, but without seriously reduced capacity, would make it the best cup for me.