There are a couple of sizing measurements to keep in mind when looking for the best MTB cranks. First, there is the length of the crank arm. Crank arm length can impact riding style and bike fit, but most riders just run what has come pre-installed on their bike. It's good to keep in mind that longer crank arms run the risk of striking rocks and trail debris with your pedal or crank arm. This rarely damages the parts to the point of disrepair, however, it is not a pleasant experience and can lead to crashes.