its really up to three things:
Personal preference,
Type of skier you are
And where you will be skiing
So for me, i am an all mountain/ backcountry skier, so i dont go into the park and really want a well performing ski, so i usually go longer. So i have 185 ARV's and 186 Comis (because they only come in that size is the main reason, but anyway)
But the sizing usually goes like this:
Beginner = ski tip at the chin
Intermiate= ski tip at nose
Advanced= Ski tip at top of for head (where my all are)
But based on the assumtion that you will be in the park, i would get a 170 ish, but no longer than a 175, and no shorter than a 165.
Let me know if you have anymore questions
Ferrari