Sizing is tricky but the gaiters are perfect. - Outdoor Research Crocodile Gaiters - Men's

Like others have mentioned, figuring out the right size involves a little guess work. I wear a size 10 Merrell Glacier Shell boot but I have very skinny legs and the circumference of my calf is only ~15 inches. I decided to try my luck and size down to a Medium based on the skinny legs and calf. Happy to say the medium worked out perfect!
Covers all the way around my boots and stays up on my leg but isn't bulky or bunched. I primarily use these for snowshoeing and it does an excellent job at adding a little bit of warmth while keeping the snow and water out of my boots. Highly recommend these gaiters!