Your overall body size will influence what length you need for a comfortable fit or the style you want. This model is 5'5" and an average/slim build. If you have a larger/smaller neck or bust, or longer/shorter torso, you will have to make adjustments accordingly as the lengths in the image will fit differently on you.
For a more accurate and comfortable fit, measure the length you want on yourself with a measuring tape, or use a piece of string, marking the length and then measuring the string with a ruler.
Remember too that pendants will add extra length. General necklace lengths are given with the additional pendant length indicated.