Replying to Ski size issue/mounting

so im like 5"8ish and im riding a 171 next season (ar6).

im only like 135 pounds and a HS senior so not going to grow much i was thinking i can trade in my planks for a smaller size they are about the same size as me now. i could go for the 166s i think they are armada makes, i had 169s last year and i kinda felt i couldn't make the spins like i use to on my 159s the year before. my worry is that i will not be able to spin as easy with the 171s

im going for like 75% park/25% all mtn

the next question is IF i stay with the 171s where should i mount center??