Good Afternoon Camilla
Thanks for your enquiry about the leg guards and it does vary from skier to skier, some prefer a guard that protects the knees but only has a modest overlap down onto the boots whereas others like them to give coverage from the knees almost down to the instep of the ski boot.
The only thing to really consider is that if they are too big they will feel awkward and inhibit the skier's free flowing leg movements.
All in all, I would say that it would be Juniors for anyone underr 1.55m and the Adult ones for anyone over 1.60m. Between the two you could really go either.
I know this isn't a definitive answer but hope it does at least give a couple of pointers.
John and Team at Snowfit/Revolutionz