Apparently I have a medium size wrist and both the large and the small sport band fit. I noticed that the holes in the two bands are not in the same places, so if you find that you can't get a good fit with one band then try the other.
For me with the large band it was just a bit too tight in one hole or just a bit too loose in the next hold, but on the small band there's a hole that's right in between those two that fits much better.
For me with the large band it was just a bit too tight in one hole or just a bit too loose in the next hold, but on the small band there's a hole that's right in between those two that fits much better.