What Size Water Ski Should I Buy?
Along with Boat Speed, the build of a skier (height and weight) helps us get the right size ski. The amount of ski that is needed by a pro-football linebacker is going to be completely different from a 100 pound kid. Slalom skis are designed to stay on top of the water, they do this with surface area. If we have more weight to support, we need more surface area. WaterSkis.com puts a suggested weight range on all of the skis we offer. We do this to each ski specifically because each ski shape and design will change the way the ski will ride and therefor the intended size for that ski. Here is a screen shot of our site showing several ski sizes offered and their corresponding suggested weights:

Sometimes you may want to choose a different size ski. This is a very situational adjustment. Here are a couple of examples. You are buying for a 140 pound 15 year old boy that is already close to 6' tall. Based on weight alone we would typically recommend getting a ski that is 65" long (based on chart above) but because of the height of the skier and the fact that he is 15 and will probably fill out his frame you would want to go with a 67" ski. If you trying to chose between two sizes, keep in mind that a longer ski will carry more speed and be more stable, a shorter ski will turn quicker, but will fatigue the skier quicker. In general we are recommending much larger skis than we did 10+ years ago. This is because newer skis can turn better, even in larger sizes, and there is more stability and speed between turns to be had in larger skis.
With these three factors in mind you can get the right ski the first time.
More info on choosing the right water ski.