YOUR ALL FUCKING PUSSIES!!!!!!!!!!!!!!!!!!!!!!!!
170 is the lowest you should go, who cares if you can spin 1/16 of a milla second faster, landing and shit, plus sking the rest of the mountain is way easier on longer skis.  Plus because of the rear tip, the skis will like like 5-10cm shorter than a non-twintip, but there is that extra weight, which is good for balancing your spins.  Take it from 5'9 Tanner who skis on 180s and 5'9 Candide who skis on 181s, and basically every good skier out there.  170 is the lowest you should go, i'd advise going for 175-180s, you'll thank me later.
My Skis go both ways... Like a Bisexual