Ski sizing is arcane at best- 170, 201, 183- the numbers might as well come from a magic eight ball. And to some degree, it is arbitrary- you could probably make it down Stump Alley on a pair of 223’s or 150’s. But the odds are good that something in between will work way better.
Over time, skis have shrunk. Gold Country miners raced on ten-foot long planks in the 19th century. Standard ski size for decades was as high as you could reach- 210 cm for most men. Today, professional freesking, World Cup Giant Slalom and mogul skiing are all skied on 185’s.
Technology circa 2004 essentially squeezes a long traditional ski into a new, wider and shorter shape. A modern all-mountain ski has all the mass of an old 207 GS ski and just as much edge contact- but it’s a foot shorter so it’s easier to hop around. It has almost 50% more surface area, so it skis soft snow better. Its been designed and tested on a computer, and the construction integrates a variety of carefully sculpted materials in a sophisticated package that absorbs shocks and vibration. Binding integration allows the ski to flex freely, conforming to the terrain, increasing edge contact and grip. The overall flex is softer, easier to ski and more forgiving. The deep sidecut initiates effortlessly and pulls you through perfectly carved turns.
That big old GS ski still might have the edge in straight-line stability on hard snow (only because of is straight sidecut), but in every other aspect, the new ski wins.
The size range is what it is (150 to about 195cm) because the smallest sizes are about right for the smallest skiers, and the biggest sizes are about right for the biggest, strongest skiers (and the chronically insecure). In general, sizing today is pretty simple- ski length should be about your head height- a couple inches more if you like more speed, a couple inches less if you are more conservative (or prefer short turns). This will put most men on about a 180, and the ladies on 160-165’s.
Going to the next longer size will add stability and smooth out the ride a little, at the cost of low-speed maneuverability. When you drop a size, the skis feel more responsive, but less stable at high speed. The bottom line is that you should ski the shortest size that you’re comfortable skiing fast on. Anything longer is just unnecessary extra effort, and using skis that are too big can force skiers to develop awkward technique to compensate. The skis should be short enough to turn easily at the speeds you ski at, and long enough to feel stable when you open it up.
If you can, try different sized skis for yourself- get a feel for what changing length does the way the skis (and you) work. You’ll learn something and its good for your technique- going from long to short (and vice versa) forces you to re-center and adapt.