Remember, the vent size is smaller than the lead flashing around it. A 1 1/2 inch vent will have a 2 1/2 inch flashing, a 2 inch vent will have a 3 inch flashing, and a 3 inch vent will have a 4 inch flashing. So when measuring the diameter of the vent, if it has lead still around the top if the diameter of the lead is less than 3 inches use a Size A, if its 4 inches use a Size B. If its 5 inches or more than its the rare 4 inch ven which we dont make a shield for.