“Someone may feel that a specific number is arbitrary, but there has to be a line somewhere.”
Im not sure There HAS to be a number. As a kid We couldn’t have a yarmulke that was “too small” the Rebbi decided what was too small and quietly told those who needed larger one. there was no rulers involved and we werent lined up in front of our peers to have them literately shatz up how frum we were.
Rebyid put it best ” Whether or not the hair length is proper depends on how it looks. So look.”