Get a cloth tape measure and wrap it around the leg at the middle of the kneecap while standing. A measurement of 10 to 12 inches fits you in an extra-small pad; 12 to 14 inches is small; 14 to 16 inches equates to medium; large measures 15 to 17 inches; and extra large is 17 to 20 inches.
How tight should knee pads be?
Do you? It should fit snug, but not tight. You don't want it to slide when you dive or bend too deeply, but you also don't want it to cut off your circulation. It should be comfortable enough to wear for an entire hour.