If you assume that 170mm is normal for an average male 5'10" rider then you can scale the crank length to some proportion of leg length (often femur length) to get a normal crank for any other size rider.
The reasons for departing from "normal" size are several:
You might use a shorter crank if:
You want to spin faster.
Ride a banked track
Your bike lacks sufficient ground clearance.
You want to avoid toe-clip overlap with the front wheel.
You might use a longer than normal cranks for:
slow, hilly off roading.
If you prefer to apply lots of force to the pedals.
Crank length affects the torque at the bottomk bracket but has no effect on the speed (at constant power). Longer cranks rotate through a longer distance so require more work per rev but each rev takes longer.