I've got a pair of these that are still soft and cushy after one year, so ordered these for Mom for her birthday. Mom's feet are long and very narrow (11AA) and you can imagine the problems she has finding shoes that fit. These Acorn slippers are perfect for her. I'm a 10A and my pair is also a perfect fit for me. Based on that, I'd say these run a bit narrow, somewhere between A and B. The length is spot-on.

Some thick foam slippers are poorly constructed and cause the foot to slip off to one side or the other. I'm not sure why this is, but found it to be true when I was shopping for my slippers. That is not an issue with Acorn Thongs or Spa Slides. The foam "strap" keeps my feet nice and toasty in our mild Florida winters. Highly recommend!