bigger = more stable at speed, faster, and harder to wash out on jumps
smaller = lighter, faster spinning (for those future spins) and easier to butter or wash out.
that being said, i am 5'7'' and 150 lbs and i ride 172s
i like the stability on landings, and even on handrails. also if it starts to snow, and you left your pow skis at home, its nice to at least be able to still shred fresh snow. skis too short can be frustrating in even 2" of new.