I tried on 17, 18, 19 & 20 today. The 17 fit the way Cartier Intended (the way a handcuff would, for lack of a better description lol) fitted, but not tight; 18 had a little more room. .. Was able to slip both 19 as well as 20 over my hand. My wrist is 6 and 1/4 " for reference. Hth
It depends on your wrist shape. I found two sizes to be the exact same width but the bigger size was taller. My wrist is wide and not tall if that makes sense so the two sizes could slide up my wrist the same but the bigger size just stuck up more on the top and bottom which I didn't like