Can you please help me find the right bracelet size?

Okay long story short… I’m getting my best friend a bracelet from Tiffany and I can’t seem to know which size to get. I can’t ask her because she’ll figure it out and I want it to be a complete surprise. I’ll be shipping everything so I really have to pick the right size. We live in different countries so I can’t really find out. Ugh, if I only knew her wrist size.

I want it to be perfect and I have no idea what to do. This is a picture (was doing her nails) I found that might be of help? Another one.

The bracelet that I plan on getting has three sizes – Small Medium Large.
Flutherites, help.