I am 6'1" 170 lbs and ride a 171 invader (center mounted, naturally) in the park. Most people would say that is too short, but it works great for me. In fact, my 6'5" 190lb brother rides my old Chronic 172s, and he rips just fine in them.
Long story short, few cm shorter or longer (+ or - 5 cm or so) isn't going to affect performance greatly.
168's will be more than enough of a ski for you too learn on man. how old are you? becasue you even have some room to grow or if u are done growing you should still be fine. pm me if u have any questions.