Reasons to get a shorter alpine ski:  You are a beginner or intermediate skier.You weigh less than average for your height.You like to make short, quick turns and seldom ski fast.You want a carving ski with only camber, no rocker.
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Reasons get a longer alpine ski:You are skiing fast and aggressively.You weigh more than average for your height.You plan to do the majority of your skiing off the trail.You plan to ski a twin-tip ski.You want a ski that has a lot of rocker.

What about camber and rocker? Width, camber and rocker also play a role in which size/type of ski will work best for you and the terrain you ski. Check out this article from EVO to learn more.    

One of the more confusing parts of buying Nordic skis & boots is making sure that your boots and bindings are compatible. Not all boots will fit all bindings. Plus, you'll find different boots and skis for different types of cross country skiing (skate skiing, classic skiing, and even backcountry touring).

For example, New Nordic Norm (NNN) boots/bindings will not work the Saolomon Nordic System (SNS). Make sure your boots work with your bindings! Volunteers on the sales floor may be able to help you!   

Here's some helpful guidance BSF gives parents looking for Nordic skis for their kids:

CLASSIC SKIS Ski tips should reach the wrist area when the arm is held above the head. Poles should reach somewhere between the armpit and the top of the shoulder (but not above the shoulder).
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SKATE SKIS Skate ski tips should reach between the forehead and top of head height. Skate poles should reach between the upper lip and tip of nose.