I bought a Large based on the sizing guide. It fits the shoe (Giro Empire - size 44) perfectly, but the area that is supposed to grip the calf is too large. There is 10mm gap all the way around which leads to rubbingdiscomfort and allows water in which defeats the purpose of having a gripper there. While I do have skinny legs, the product should accommodate a reasonable size range...especially when using stretchy materials. The cover is very tight on the shoe, so downsizing was not an option. I had to spend a few hours with a sewing machine to change the zipper position.
Otherwise I am happy with the product now that I have modified it to fit.