I'd check on the website of the manufacturer. They should have some kind of guide on their. I think a 9 is probably a medium, but that's a guess. It's more about your lower leg size rather than just the size of your boot. Depends on the boot too. My gaiters work fine with my Garmont plastic ski boots and I think mine are mediums.

If you get an overboot like a Neos or one of the heavy duty insulated ones for climbing, then foot and boot size are important to match.