This used to be so easy. You'd reach your arm up in the air and the tip of the ski was supposed to just touch your wrist. Never really made sense to me, but there it was.

The more question is how much you weigh and what you want to do with the ski.

Classic Striding
Let's start with classic cross country skis. These skis should be like springs. Weight both skis equally and only the tips and tails should contact the snow. Put all your weight on one skis and the kicker (that area underfoot where you apply kick wax or have fishscales) should now be in contact with the snow. This is what's known as a double camber. This allows you to alternately grip and glide on the snow. But how do you figure the flex of the ski when in a shop or ski swap? 

Try lying the skis down on a flat section of floor. Then have a friend try to slip a piece of paper under the skis while you are standing on them both weighting them equally. The paper should slide under the ski until just about the heel and up to a foot in front of the toe. Any less than that and you will have too much of the kicker on the snow during the glide phase. Then place all your weight on just one ski. You should still be able to slide the paper under the center portion of the ski, but the pocket will be much smaller.

Again, the idea is when you put all your weight on one ski you grip the snow and with both skis equally weighed, the center portion shouldn't touch, leaving the tips and tails to give you good glide.

Skaters
For skaters the same thing is true, but since they use the edge of the ski for kicking off the snow instead of wax or fishscales, skating skis are shorter, but with more spring. Again, the skiers weight is the best way to fit the skis, but how many of us can test the flex of a particular pair of skis?  If you want a really good pair of skis I highly recommend visiting a first class shop like Sierra Nordic in Soda Springs or by mail-order through Eagle River Nordic. These people test the flex of the each pair of skis they sell to make sure you get the right pair to fit your needs and weight.

Telemark Skis
For tele skiers it is a little more confusing. Weight is once again the main factor, but for different reasons. With downhill and tele skis, the center portion is in touch with the snow nearly all of the time, allowing you to turn better. 

So the goal with tele skis is floatation. On the hardpack of a groomed downhill area, you would need less flotation than the fluffy powder or breakabe crust of the backcountry, so you could use a shorter ski. Shorter skis are easier to turn, but not as stable as you get faster. Plus longer skis tend to float better in crud conditions. There are a lot of tele skis on the market today, many of them rehashed backcountry boards that just don't handle well on the downhills. But the latest crop of tele skis are starting to look real good.

And because the newer boards have more sidecut the shorter skis tend to have more surface area and therefore will "float" better in softer snow.

What length to choose? There is no clear answer. I've skied everything from 180's to 205's and the 180's were too short and the 205's were too long. But it took me a while to realize that with both lengths. For your first pair consult a good backcountry ski shop and get advice from them. There opinion is as good as mine and I haven't got one. 

Backcountry Skis
And if you want skis for backcountry travel and not just going downhill, floatation become even more important. There is one major difference between a tele ski and a backcountry ski. Backcountry skis normally have a double camber (see the section on Classic Striding Skis) so you can grip and glide better. The trouble develops when you go downhill. That middle section of the ski is not in good contact with the snow, so turns can be a bitch. Tele skis on the other hand do not have much camber, so while they are great when it comes time to carve some turns, they can be a real slog to travel cross country on (poor glide). 

Nothing's easy is it? Again, the key here to the correct ski length is floatation of the ski related to your weight. Get a ski that is too long and the kicker will never get good contact on the snow and you will be slipping all the time. Get it too short and you will get good grip, but that same grip (be it wax or fishscales) will be dragging on the snow all the time slowing you down (and quickly abrading the kick wax off the skis).

So can see, you should leave room in your garage for at least three pair of skis, if not more.