13" calf/Thigh what size knee guard?

My leg is 13" in diameter for both the largest portion of the calf and the lower portion of the thigh, basically where the knee guard would go. I am looking for a knee guard that straps at the top. Do I need to get the adult size or the youth size? I wear a youth large chest protector if that helps but cant figure out which size knee guard I should get. I dont want one thats gonna slide move around because its too big all the time.. Thanks!!

Damon