The simplest way to size a ski pole for traditional alpine downhill skiing is to flip the pole upside down so that it's handle touches the floor. Grip the pole with your hand under the basket. Your elbow should be at a 90 degree angle when you do this, if the poles are sized properly.
For cross-country skiing, where the poles are a primary part of the skier's movement, size the poles so that they come to rest just under your armpit.
Skate skiers, who do cross-country racing with a motion similar to ice skating, use slightly longer poles. Generally, a pole sized close to the base of the nose from the ground is right, though there is leeway. They can be sized between the chin and the your nose.
Telemark skiers, who ski downhill in nordic equipment, need a shorter pole, since the telemark stance puts them lower to the ground. Generally, the pole length should be about the height of your navel.
Some backcountry skiers like to get adjustable length poles, lengthening the poles for traveling uphill and shortening them for traveling downhill. The same general length requirements for alpine or telemark downhill and for cross-country apply.