Based on other reviews noting they run small, I ordered the XXL as my shoes are a size 45. If I needed to fit on my road shoes, the Reflex covers would likely fit, but . . . since I intend to use these in the cold-freezing weather, I use mountain biking shoes for the additional traction. These covers require significant effort to pull over the rubberized tread and nearly impossible to pull over the heel. So while I was successful once, the second time I spent 5+ minutes trying to get one on but gave up as I was due to meet up with my fellow breakfast riders and did not want to be late. Instead, I had cold toes for the entire 2 hour ride.
These are really well constructed and would keep your feet warm. Just order a few sizes larger than what you think you need.

These covers work great, especially for price point. Warm and dry. I also like the fact they aren't covered with logos.

I wear a size US 11 (45) shoe. XL's did not fit. XXL's fit fine. So size up!