I had similar questions when I first purchased the diva cup. I also had no contractions prior to my c-sections. Because there are changes in the cervix and other parts of the body during pregnancy even without a vaginal birth, I decided to purchase the post-birth cup and it was the right decision for me. I hope that helps - good luck!
The cervix in a younger (pre-childbirth) woman is very round. The cervix after childbirth is more of an oval shape. The shape of the cup needs to fit securely over the cervix in order to prevent leakage and to feel comfortable.
I bought them both. First I bought the age-appropriate, larger size. It was uncomfortable and difficult to remove causing more and more discomfort over the several days I needed it. I decided to take a chance and bought the smaller size because overall, I like the product concept. The smaller size is MUCH more comfortable and removal is not a problem. It still fits and does not feel as if it will "fall out" or anything like that. it's only slightly smaller, (more tapered,) with a slightly smaller ring at the top.
I think in that case, it would go by your age... Which I think is before or after 30. But if you contact the company, they would likely be able to tell you for certain. You could probably also ask your doc, could give you a better idea of what size you'd require personally. Cheers!