I had a hair check a few months ago as a pre-employment check for a job with the police.
They just took what they could and made do, there was no mention that it had to be a certain length.
I did see something on the paperwork that they could have used body hair if I'd had a shaved head, I guess it's not uncommon that they have to find a workaround for people who are bald or have alopecia.