This is so that I would be recognized from my passport if I was detained for several months in a sketchy country.
In one case, I was walking in a shopping area, someone approached me looking to sell souvenir artwork, I explained that while I didn't need that I was looking for a haircut, and she offered to take me to a barber provided I bought a picture. Everybody wins.
He would've likely double-checked with me if I tried to spell this out at the shop[1], but apparently I came across as someone who really knew what they want, coming in confidently with the order already precisely written in Chinese and all.
Lesson learned. I still think the idea was good, and I'd still go for giving explicit length (it's a natural fit, as it translates to cutting head numbers for the electric hair cutters). I'd just triple-check it next time, and not act like I have it all figured out.
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[0] - Or thereabouts; I'm sure about the unit, but the exact number might've been something else between 2 and 6.
[1] - The barber didn't know English, but knew the metric system and arabic numerals, so we've confirmed the misunderstanding with pen and paper.