ok if i recall correctly 661 gives top and bottom opening sizes, the more important one is the top opening, what you wanna do is take a tape measure and measure around your leg right above your knee (if u don't have a soft tape, use string and then measure the string on a yard stick or w/e) bottom opening is the size of your ankle
^^ It would be helpful if people who have actually WORN SixSixOne gear respond to questions like this.
Hulk - - Your natural instinct, at your size, might be to go the XL; BUT, every 661 guard I've ever put on is large for the marked size. I order medium everything (6'-1", 175#) and I've got every strap cinched as tight as the velcro will still hook. It's all on the roomy side. I've only worn one 4x4 item (elbow/forearm); but gauging by other 661 gear sizing, I'm going to say the Large will most likely be plenty.