I've been using DivaCup size S for about 6 months now and have never had a cycle where I didn't leak. I have to wear a pad for the first 4 days, but the other 3 after I don't (hurray for that at least!). I have a heavy flow and have to change the Diva every 3 hours (it'll be at the 15ml line) the first couple of days. Although I don't think the capacity is the issue for leaking (not sure what it is), it's probably best to get a bigger capacity anyway for the heavy days so I don't have to change it as often during the day.
By looking at the sizing chart, it says this:
Next period, reach in to feel how long your vagina is.
If you can reach the end (not necessarily your cervix) easily, it's on the short side.
If you have to reach really deep or still can't reach it, it's long.
If it's in between, aim for a medium length cup.
I'm not sure how long my vagina is as I have short fingers, so even if I can't reach, I have short fingers.
I can touch my cervix and my finger is 70mm (less than 3 inches). I don't think I could feel the end, maybe because my cervix is dangly during the heaviest flow.
I'm not sure if I should pick short, medium, or long? Any recs for a heavy flow?
The Diva is comfortable to me, I don't feel it, the stem doesn't protrude, and I can just grab it with a little bit of bearing down. I'm not really sure what I should be looking for in the size charts.