Most people can comfortably use both sizes, but one may be a better fit than the other. Some things that you might want to consider is your flow and heigh of your cervix. The most important thing is not to overcomplicate things. The only way to find out what size is best for you is to try one.
The small size YONI is suitable for light flow and is a great beginner's cup. The regular size YONI holds slightly more fluid and can, therefore, be better for a heavier flow.
If you have a low-sitting cervix, we suggest the small YONI as it is slightly shorter.
Some other things that you could consider when choosing a size are:
Lifestyle: If you're living an active lifestyle, or are a kegel gold medalist consider using the small YONI.
Age: For teens and younger menstruators, the small YONI can sometimes be easier to insert.
Pregnancy: If you have had a baby, a regular-sized cup is sometimes a better fit.