Weight is the predominant factor, but skiing speed may also be important. For recreational skiers there may also be factors such as boat horsepower. How much you weigh plays an important role. Many manufacturers use weight alone as the guide for ski length. But of course this varies depending on the model of the ski.
Many manufacturers make larger or smaller skis for larger or smaller skiers.
How fast you ski also plays a role in ski size selection. If you are a recreational skier that skis at 26-30mph you might consider a longer ski. Other Factors There are many other factors that can affect your decision about ski size. A table in the 1994 HO sports ski catalog provides most of the information you might need as a general guide to ski size selection. It is always best to check the information provided for the actual ski you are considering buying for the most accurate size selection.