To me, generally, high speed stability resulting from a longer ski means that the ski does not react to the terrain as much as a shorter ski, and feels planted, when at a fast speed. For example, when I ski a shorter ski on groomers I find myself overpowering the ski and feel the ski reacting to the terrain too much such that I am not comfortable pushing the ski harder.
In the park context, the stability provided by a longer ski allows me to take speed into a jump with the confidence that the ski will stay planted, and not chatter/be thrown around etc, as I approach the jump (also higher top speed). When landing, the longer ski gives me more area to land on in case I'm not perfect on my landing. When landing quickly the longer ski gives similar stability as when approaching the jump. Given your size, I could see you landing a bit off and easily washing the tails of the short ski out - certainly proper form is important, but constant pain through falls on unforgiving skis is not worth the lesson in form IMO.
Between 169 and 179 is an interesting question. Note that K2 sizing is off compared to other ski brands, so a 169 is more a 172 and a 179 is more 182 (give or take a few mm.)
You're not a very tall rider and your described skills do not necessitate the biggest ski you can find. Two views: You can always grow into big skis, I enjoy this part of skiing personally, and 179 is a ski for a rider your size. If you do fear a big ski, and unless you see your progression rate soaring both in and outside the park, you should be fine with a 169. Ultimately, given your relative inexperience in the sport (from what you've said) i would go with 169.
Hope that helps