A non-contacting bale size sensor for an agricultural round baler having bale-forming chamber partially defined by a plurality of continuous side-by-side belts trained around a plurality of fixed guide rolls and at least one moveable roll connected to a belt take up arm. The bale size sensor detects the position of the belt take up arm by measuring distance between a fixed position and the take up arm and a connected control unit determines the size of the bale within the bale-forming chamber.