There are a lot of factors that can influence your ideal ski length. Getting on the right size ski is an important part of being comfortable and enjoying your time on the slopes. Using the skier's height is a good starting point and taking things into consideration like weight, ability level, terrain, and even their personal style of skiing will help to narrow down your size range. For most people their starting point will land somewhere between their chin and nose. Remember, these are just guidelines, not rules. If you have a preferred size that you are comfortable on you can certainly go with that.
Beginner: If you are a beginner looking for a ski that is easy to maneuver and keep in control, consider sizing down 5-10cm from your starting point. This will usually put the ski at chin height or a little below. This is a common practice at resorts and rental shops as it helps to build the confidence and essential skills a skier needs to move beyond the bunny slopes.
Intermediate: Are you an intermediate skier who has mastered the basics but is still progressing? Choosing a size that lands around nose height will give you the length needed for stability on intermediate terrain but keep the skis from being too unforgiving when mistakes are made.
Advanced: By the time skiers reach an advanced level they usually have a good idea of what they need in a ski. A longer length is usually preferred as conditions become more demanding and the additional stiffness and stability are necessary. This will typically mean a ski length that is about forehead height.
Conservative: If you prefer to stick to the groomers a shorter ski may be for you. Keeping your turns short and going at a moderate speed means that a ski at, or slightly below, chin height would more suited to your style.
Aggressive: For skiers that just wanna go fast. If you are spending most of your time skiing at high speeds on steep or technical terrain, selecting a size that is closer to forehead height or slightly above will give the added stiffness and stability that is needed for this style.
Groomed: For skiers that spend a majority of their day on groomed, hard packed, or icy surfaces, going with a standard length is best. A standard length will keep the skis maneuverable and for that extra bit of responsiveness look for something with a normal to narrow waist width.
Powder: When you spend a lot of time in fresh snow or off trail, a slightly longer ski will give you that extra bit of float to keep you moving through the deep stuff. You will also want to consider a ski with a wider waist width that will also help you float and keep you on the surface.
If you are above or below the average weight for your height you should adjust your size accordingly. If you are above the average weight for your height you can consider a longer ski for added stability and response. If you are below the average weight for your height you can go with a shorter ski to get the flex and maneuverability you need.