Whenever I get this question in the shop I have the what kind of skier are you / how aggro / etc talk.
I will say this, if you ski real aggressively and want to charge with stability at speed go with a ski your size. If you are looking for something park specific, more specifically rail specific a shorter ski CAN make certain tricks (like switch ups) a bit easier. I personally don't find this to be the case. I ride in the 170-75 range for an all mountain ski and I am 5'8".
168 would be real squirrly for me at speed, and just overall not confidence boosting when maching into shit.
Seriously think about getting a longer ski it can help you progress.