My wrist circumference is approximately 165mm and I'm getting the 42mm model. I've seen a lot of posts on these forums where people have posted that for a wrist which is thin as that the 38mm is a better option. I have to disagree. There is no wrong or right choice based on wrist size in my opinion. At the moment I wear a Pebble which is quite big itself and it looks just fine on my wrist.