At least 175. 165 would be so unstable. Unless you're planning on only hitting rails and nothing else... actually scratch that, go with at least 175, you won't regret it.
at 140/150lbs 175 will be long enough, however, you could ride a 182 fine as long as your skiing ability is decent. For all park i'd say 175, if your going to use it for alot of all mtn, i'd suggest you get the 182.