When you've decided on the type of ski you want, you then have to choose the right length. Once upon a time, as an adult beginner, you would start on something around 150-170cm.
As you grew more confident, you would aim for a ski your height plus 20cm. A bit longer if you liked speed, a bit shorter if you liked ease of turning.
With the invention of carving skis, things have changed radically. The new skis are taken up to 20cm shorter than traditional 'skinny' skis. Short is now cool. Even racing skis are shorter and shapelier.
As a general guideline most carving skis should reach between your nose your forehead. However you need to consider the type of skiing you want to do, your level and your weight; a ski can't tell how tall you are - only how heavy you are.
If you are a beginner or like to ski slowly, then go shorter - for ease of turning and control. If you want to ski fast and hard, turn little and have better stability, or if you want extra flotation in the powder, then go for a longer length.