Long cranks enable you to turn larger gears at a slower cadence. With small cranks, you can turn a smaller gear, faster. In both cases, you are outputting the same amount of power and going the same speed. Cranks do not increase your power.
Long and short are relative terms. The "normal" size depends on your leg length. You also need a crank which will not interfere with the front wheels, and which gives sufficient cornering clearance.
For ave male (5'10), a 170-175 is about right for off-road riding.