To me, short hair on a woman is down to the bottom of her earlobes; medium is slightly past her shoulders and after that, it's long. So if a ponytail is anywhere near her shoulders or reaches past her shoulders, she has long hair. It's so subjective that I had to set my own standards.
In the absence of any guidance from an MSC, I decided awhile ago that only women with medium and long hair make buns so I just look at the size of the buns to report if it's medium or long. I often report a female with a bun or a ponytail, whether it's high or low on her head, as having long hair. I don't use "medium" very often. It has to be a very small bun to be a medium to me.
As for men, unless a guy is bald, men either have short hair or long hair. I've never used "medium" to describe a guy's hair. Don't know why.
I used to have problems describing men with facial hair (when writing a narrative) until I just decided to use "facial hair" without describing the type (moustache, goatee, etc). I was questioned only once on an evaluation about a bartender who had a very light chin-strap that most people could barely see in the dim light. But he had one so I just said he had facial hair. The report was accepted after I explained it was a chin-strap, not a full beard.