my pow skis are 20cm taller than me and my park skis are 14 cm taller than me. are they too big? psyche! it's all personal preference. longer skis will give you more to land on and in my opinion they spin smoother. short skis just suck. i wouldn't ride anything that isn't taller than me, but i also know people who hate long skis.
just ride what you have and try different skis to see what you like better.
I think it kind of depends on flex, but is mostly preference. I would never buy a soft-medium flex ski that is shorter than me. But i recently bought a pair of TJ's that go to my hairline and love em. Nice and stiff, really stable on big jumps and speed. Overall bigger is better. IMO