based on the sizing guide, I went with a large because I was on the medium line. it fits a bit loose where I usually wear a watch, but I think it's OK. it doesn't have any problem getting a pulse.
my wrist measures 6 7/8" (175mm) just up the arm from the Ulna "bump" (on the outside back of the arm). if my wrist were any smaller or one wanted a more firm fit, I'd go for the medium.
I wish Microsoft had given actual measurements as size ranges instead of that visual guide. :straight: