I'm 5'5" and I ride in CO and WI.
When I'm in CO I ride 171's... More powder, bigger features, steeper runs, etc.
When I'm in WI I ride 151's... Completely packed, smaller features, smaller runs, etc.
It's personal preference, what terrain you're going to be riding, what ski you're going to be riding, what ability level you're at... etc, etc, etc.
Like everyone else has mentioned, the longer the ski the more stable they are... but longer can make spins and rails more tricky.
If you're comfortable on the skis you're on, then stay on them... don't let anyone tell you they're too short or too long.