I have size 12 pac boots (huge at bottom) and I use XL size and they work great. On a sidenote, I've broken 2 pair of kahtoolas with normal usage for ice fishing only...so i'm switching to the hillsound brand this year to see if they are any more durable. I bought them from amazon.com for $52 shipped, and they have a strap that goes across the front of your foot to help keep them in place better.