Actually they do say the size is different... Here is what they say.
New from AFX Manufacturing! Underdrive Pulley to free up horsepower by reducing rotating weight and slowing down the speed of ALL of the belt driven accessories. Comes with new belt. Crank underdrive pullies such as the AFX are much more effective than others which only slow down the alternator. You get more bang for the buck with the AFX UDP! Measures approximately 3.5 inches in diameter. Please select model year as the belt size is different starting in 2003! If you do not have air conditioning, please call before ordering. Kit includes one CNC machined aluminum crank underdrive pulley and drive belt.