Guys I feel like I have "small wrists" for a guy (since I'm slender) but not as small as some people.
I have an iPhone 4S test device here and that is 58.6mm wide. That just barely covers my entire wrist if I put it over it. The 42mm watch is 2cm smaller than that.
Personally the 38mm modal is only half a centimeter smaller, but the 42mm makes the watch look thinner by comparison. I'm going that route since I'm a guy and I don't want some puny watch on my arm.
Could I get away with a 38mm? Yeah sure, but it's the smaller version.