The sizing is accurate, and to fit over my wetsuit I chose a large. They are hard foam/plastic inside as the padding and have 3 big velcro straps in back (no slip on). I've personally tried 2 other brands of knee/shin guards and both of them slipped down when you recovering from a jump, and water rushes under them. The brand I sent a link for are the only ones that don't slip down.
I wear them when i'm on my hurricane. I think short flatwater hulls with forward footholds tend to mangle knees and shins since you squat down/ knees forward. I did get a few dings in my knees on my fake-ter riding Pismo surf without them. I use the small hard plastic ones under my suit. over the suit and they don't stay in place very good. here are some of my collection