I wear a 9 1/2 myself. I've got a pair of the Arctic Shield boot covers in the next bigger size, like 11-13 I think and these fit my Lacrosse Expedition pac boots about right. So, If I were you I'd go up to the next bigger size. Getting them too snug will help to defeat the purpose. Loose fitting is better.

By the way, I've only needed mine one time. I was on stand last year for about 4 hours when the temp was about 10 degrees. I got pretty chilly, but my feet never really bothered me. I haven't needed them this year yet.