Judging on your profile this is your 2nd season... so i would stay in the realm of 179 to 183 or so. Maybe even 177 if you're just beginning with park. With your height, getting into the lower side of 171 things are going to get a little squirrelly and somewhat difficult when you venture into pow.
Here's the thing, with long park skis, you might find yourself getting hung up on boxes rails more than a short ski, plus the learning curve on super long skis is harder. But you don't want to go too short.
I'm 6' and anything under 170 just feels a little squirrelly and weird to me, but that's just mho. Glad to hear you switched over from boarding man... hope this helps.