when ever buy any kind of slippers i always go a sz up. When it arrived, it looked rather daunting with all the tools and instructions. Avoid this mouse even for casual use it is completed unfit for purpose. Also Ive done some vibration tests and the microphone stand that comes with it doesnt always pick up my bumps on my desk. Bought as a gift for my son in law, bought his size but he said the fit is a bit snug. Sleep tracking not good for me. I really like the tap and swipe functions. Installation was a snap, as out of the box it fits any standard round 31. So, I gave them a try and ordered a 10 and 11 (usually wear a 10. It does both beautifully.