Michael-Scott E.wrote a review of Demon Dirt Davey Jones Knee Pad - Men's on July 29, 2011
Measured around the knee. Got sized for Large and they fit perfect. I ride XC and don't even feel them after rides of over 20 miles. They do get hot, duh, but they've saved my knees from some good thrashings.
Also- I bought these for under $50 here at H&R. Now they are $70. At that price point I would have gone with the POC's or the Fox's instead.