Im 5'10 and i ski this years 177 Stl and there a good bit above my head big skis are deffinatly better for everything. I find with little skis like 170 or less the landings start to get sketchie and jumping is weird.
Park should ideally be shorter than you, between chin and eye height. People obviously have their own preferences, depending on weight and skiing style, but saying flat out that park skis should be bigger than you will be wrong for the majority of people.
just remember god told me ski size is perfectly proportional to your penis size. and i heard it was boobs for girls but its hard to tell with ski jackets on.
i use really short skis for handrails and rock skis, like maybe my chin or lower. but it helps a lot having shorter skis for rails just cause i feel like i can sw up and spin on easier. but for park and stuff i have skis that are level with my eyes, maybe a lil higher. i generally like smaller skis