They're sized based upon the diameter of your sway bar where it passes though the bushings. So, as mentioned above, you can measure it using some calipers. No calipers handy? Snug down a large adjustable (crescent) wrench on the sway bar and then measure the opening with a metric ruler or, if you have a large enough set of metric open end wrenches, see which one fits the bar snugly. While a decent set of calipers should be in every wrencher's tool box, their accuracy is not absolutely necessary here since the sizes are in even millimeters.