Hi! I bought two Cartier watches online and am about to purchase a third, so have been in the same place you are with that decision.
It’s been a crazy day so I’ve not been able to measure my medium panthere, but I’m medium bone structure with a 16 cm wrist. The medium size (I have the ss without diamonds) is a great everyday watch for me, but since I already have a medium ss TF that serves the same purpose, I wish I had chosen differently.
I’m now planning to purchase the small panthere with diamonds, still deciding between gold and ss. The small has a much dressier look than the medium (and would look great worn with a tennis bracelet!).
In the end it comes down to how you plan to wear it. If you’re going for an evening/dressy look then definitely stick with the small and consider the diamonds.
You’ll love shopping with Cartier online. I had outstanding experiences both times. If you find it’s not right for you, you can always return/exchange. I don’t regret getting the medium because I’ll alternate it with my TF for daily use, but if I could only buy one Panthere I would choose the small.