Wiper arm length is measured as the straight line distance from the center of the pivot shaft to the center of connection to the wiper blade. The bridge width between the driver and idler of a pantograph arm is also measured “center to center."
What if there’s a bend in the arm, as pictured below? The same advice applies. To determine the arm's length, simply measure the straight line distance from the center of the pivot to the center of the wiper blade connection.