Have bought two bracelets, sizing is entirely random given that a large was gigantic and a medium was tiny relative.
The reason for buying two is that the first "popped" off my wrist one day and disappeared. I thought it might have been something that was my fault, until the second one disappeared from my wrist after a week of wearing it too. In short, the bracelet clip is an awful design.
I have had Trionz bracelets for over 10 years, but now I am £50 down without one and unlikely to buy another until they re-design them.