Ski poles are essential for cross-country skiing. Poles help propel the skier forward in the classic diagonal stride and also aid in balance.
Classic styled cross-country pole sizing is determined primarily by your height. With tips on the ground, the poles should reach comfortably up to armpit height.
Casual cross-country skiers might want to have a slightly shorter pole, perhaps fitting loosely under the arms. Faster, more experienced skiers can benefit from slightly longer poles.
If you plan on skiing off trail, consider purchasing poles with a larger basket; these will ensure that your poles do not sink too far into the snow. Also consider weight when purchasing ski poles. Lighter poles will be more expensive but well worth the cost.
Consider telescoping poles that allow you to adjust their height. With adjustable poles, you can shorten them to climb easier and then lengthen poles for descents.