On the very cool and complete new website for MeLuna : http://www.meluna.org, here is a statement one can read =
“PHYSIQUE: Small, petite women often use sizes S or M. Women who are tall and strong tend to prefer larger sizes. Body size and stature seem to have an impact on the required size of a menstrual cup.”
I didn't think that such elements could actually be taken into account, or then maybe only in a few cases :) ?
I'd assume the major criterion would be the cervix height during one's period, in particular, and the heaviness of the flow, too, which is an important factor.
But I may be missing/ignoring some point or some facts here ;) (?)