HOW TO CHOOSE THE BEST MTB CRANKSETS – CRANK ARM LENGTH
The default or standard crank arm length is 175mm on mountain bikes, and is always measured in millimeters. Smaller frames and women-specific frames can both be fitted with shorter crank arms, usually 165 mm.
Crank arm lengths range from 165-180 mm, in 5 mm increments. The short arms are better for the ascent and the longer arms for the descent.
Crank arm length should be proportional to leg length. Shorter cranks for shorter legs, and longer cranks for longer legs. Several competing formula exist to calculate ideal length, but there is no consensus.
You can probably calculate the right size from trial and error. See how you feel on your current cranks, and experiment with different sizes if they aren’t comfortable.
A professional fitting is the best way to get the perfect length. Bike fittings can be expensive, but the best fitters will leave you with a detailed, highly specific fit that you can use for years to come.