Ski length influences two characteristics about the ski, quickness (turning radius) and stability. People often confuse turn length with stability, but they are very different. Turning length refers to the radius of the turn that is cut into the side of the ski, while stability refers what speeds the ski will be able to perform at. Even a ski with a downhill side cut won't perform well in a downhill course if it won't hold the skiier at downhill speeds. Of course, stability is important for all skis, but more important for some. Slalom skis still need to be stable, but only to the speeds reached in the slalom course. Downhill skis need to be stable at the speeds in a downhill course. Why not make all skis stable enough for downhill? Then we would never have to worry about stability, right? If you make the ski too stable, i.e. able to be skied at 80 mph, then it will be so stiff that you can't flex the ski enough at normal speeds to effectively use the side cut. So, there has to be a balance. Because slalom racers don't go nearly as fast as downhill racers their skis need to be much less stiff in order to be used at the speeds they ski.
Between stability and quickness, to me the stability is more important. Here's why. You can always turn less/wider and still have fun(even though this might not be desirable for your skiing style), but a ski that won't let you ski as fast as you would like will never be fun. Even if it is the quickest slalom ski on the planet and you are a slalom racer, if it won't hold at the speed you like to ski you will always be out of control. But the ski can't be too quick, because you don't have to turn as fast as possible all of the time, as long as it is stable. The nice thing about having extra quickness is that you don't have to use it if the ski is stable at speed, but it is always there if you want it. The problem is that most quick turning/short skis are less stable at speed because they are optimized for slower skiing, so you have to give up some of this quickness in order to achieve your desired stability. Otherwise everybody would ski the shortest quickest skis and just not turn as hard when they wanted to make GS turns.
So, I try to find the length that will just be just stable enough for me at the speed I like to ski, and nothing more. Anything longer/more stable will be unnecessarily giving up turning radius/quickness. If at this length the ski doesn't turn quick enough for you then you should look for another ski. If you love to ski fast, and are looking at a slalom ski, then you should be looking at longer (if not the longest) length. If at the longest length the thing still washes out when you come down at speed then you need a different ski. But, if you are looking at a race GS ski, unless you are an expert skiier, you likely won't be able to get the thing to even bobble no matter how hard you turn. In this case you should be looking at the shorter end of the range unless you are heavy or ski very fast.
So, the only way to really determine what is the "right length" for you is to demo the ski. But, by knowing the ski you are looking at (what style it was made for, what skill level, etc.) and the skier that you are trying to size (turning preference, speed, ability, weight, etc.) you can usually narrow it down to choosing between two lengths. This means doing your homework and listening what the skier tells you. [img]smile.gif[/img]