I think the measuring is to be done at the wrist bone. I don't have a soft measuring tape nor printer so I cut out a paper strip and wrap my wrist then measure with a ruler.
It boils down to your comfort preference. Hanging bangle is not my style; I don't like the Love to sit on my palm. I am 14cm and chose 15 (though SA likes me to be with a 16).
In my country, the store is easily accessible (along the shopping street, there's even two Cartier stores within 5min walk). For your case, would you be able to exchange for size if the first isn't desired?
My choice is clear on 15 as the moment I was put on and I walked around the store, I didn't feel I was wearing it at all. For 16, it bothers me.
I've been wearing it for less than 2days now. I don't feel it at all until there were moments I was snapped out and remembered i need to put on my wrist band as I was doing chores. It already has one very superficial scratch from keeping my little one's toys.
Thank you for your advice. This thread is really helpful. I went with 16 because I don't want to feel it on. Will let you ladies know how it goes once it gets here (I ordered online.)