When sizing an octopus, where do you measure from? I understand mantle length, but when someone mentions arm length, is that tip of arm to mantle? Or is it tip of arm to tip of arm, like a diameter or out-stretched length?

well i would say arm to other arm bc arm to matle would make a bimc huge but im just shooting in the dark.

Arm length is the distance from the mouth (where each arm starts) to the tip of the arm.