For me my wrist size was 6-6.2 inches and about 15.2 centimeters. The cartier wrist sizer on me was 15.5. I went with the 17. I think 18 would be too big, not sure about a 16 but I think it might be tight.
I think it really depends on your personal preference and how you plan to wear it. I'm a size 16 and decided to buy a size 17, instead of 18, because I like how it just lays flat on my wrist, doesn't move up and down whenever I walk or exercise. It also stays put when I do errands, wash the dishes or play with my dog.

It's also very comfortable when I sleep. The screws doesn't hurt my skin. It also gives me that security that the screws won't easily get loose from constant moving.
Just get what feels right for you and your lifestyle.