When deciding if a watch fits, the common measure is to measure the circumference of the wrist. Given that most watches "wear" based on the lug to lug distance (if it overhangs the edge of the wrist, it's just too darn big). It would seem the better measure is the width of the wrist. (e.g. a "flatter" wrist will be wider, but not as thick top to bottom, and could thus wear a larger watch. A larger diameter wrist, with less width, would tend to present smaller watches better.
I'm a tiny wrist guy, (circumference of just over 6 1/4"), but I find I can wear watches anywhere from 42mm on down, depending on the lug to lug distance.
What do you folks think?