The length of the cranks arms from the spindle to the pedal. Any length will work on all bikes but there are a few things to consider; If you are purchasing a replacement crank for a child's BMX its best to measure what you are replacing and match accordingly, 10mm either way is acceptable.
If it’s for a full size BMX, the length will be personal preference of the rider. Standard lengths of cranks are 170-175mm which are great for all types of riding. Some street riders, especially those running 2 pegs at the back sometimes prefer shorter cranks for better heel clearance between their heel and peg.