How to specify the length of a short haircut in China?
nothing.

Actually there is not a term to describe the length of the haircut in China.

So, how to decide the length of the haircut? keep communicate with barber.

eg.

I: 'please cut my hair with 毛寸(is an adjective for style, not length)'

B: after a while 'Is it fine?'

I: 'shorter'

B: 'Okay, let me see'