Oh, wait, i just remembered 661 has more sizes more. You could send an e-mail to them, and they will send the measurements back to you, or there might be a link on their site with measurement, and then just match to the closest one.
It will most likely be the large though, or extra large.
But that sizing chart ignores the very important measurement of how long the shin part of the leg armour is. The size of the top and bottom leg openings is not that important, that's more of a one size fits all thing unless you have large legs.
Municycle.com has a 'better sizing chart' (http://tinyurl.com/z9tve). Look at measurement "E". I like the shin length to be as long as possible. Something that fits right above the top of my shoe and goes up to the middle of my knee.
I'd pick large, I have medium currently and the pad doesnt quite come high enough to fully cover my knee in crashes. It also depends a bit if you intend to wear them over or under trousers, I wear mine over thick jeans and the medium will only just go round my leg, resulting in alot of strain in the straps which has damaged the pads over time. I', 5' 11 1/2