It really depends on how you want them to handle.
Shorter = less swing weight, less support
Longer = more stability
If you're going to be doing lots of rails/butters, you are going to want the ski to be either short or soft (or both for maximum effect).
If you're going to be hitting huge jumps, you're going to want a longer/stiffer ski (or both!)
I personally like them long with medium/stiff flex, but only because I seldom ever hit rails.