Skiing is a great sport and hobby for people of all ages. It is important to be skiing with the appropriate ski length that matches your weight, height and skill level. It is never a good idea to go skiing on a pair of skis that are too long or too short.
If you are a beginner skier the appropriate ski length should be at your nose or under. The longer your skis are the faster you will go.
Longer skis pick up more speed against the snow than shorter ones. If you are a more skilled skier you should be able to ski with a ski that is between your nose and your forehead. If someone is using a ski that is too long they will have a tendency to lean too far back on the skis. If someone is using a ski that is too short they are more likely to lean forward too much on the skis. Either way is not safe and will hold back your true skills. It is said that people who lean back on their skis are more apt to acquiring knee problems which could lead to possibly knee surgery. Also, if you are an older skier who gets exhausted very quickly you should definitely consider purchasing a shorter pair. With shorter skis you will be able to turn easier and ski slower. Children do not have to follow the under the nose rule for picking out skis. Their skis should be between their chest and shoulder as children easily get tired legs and are usually beginners themselves.
As far as weight is concerned there really isn't a set guideline that you need to follow. It is known that heavier people usually feel more comfortable on a ski that is thicker and shorter. Underweight or average weight skiers feel more at ease on a pair of skis that are longer and skinnier. This may not be the case for everyone and you should always try them on before purchasing. Go with a ski that you really feel comfortable in as this is the most important aspect.
With a pair of skis that you feel comfortable in you will be able to ski with the correct balance and great control which will give you full enjoyment on the hills. Please feel free to check out the ski sizing chart which will help you find a great ski appropriate for you.