You can make hair longer by either painting (Extend Hairs) or scaling the hair system. For information about painting, see Paint hair follicles and nHair > Paint Hair Follicles.
The Scale Hair Tool adjusts the scale value for each follicle's transform. It scales such that scaleX = scaleY = scaleZ. (If you had a non-proportionate scale to start with, using the Scale Hair Tool would make it proportional.) The Start and Rest curves are parented under the follicle transform so they inherit this scale.
Keep in mind the Scale Hair Tool does not vary either the amount of segments or the stiffness. This will make the hair appear stiffer as you increase the length. The relative stiffness of the hair will be the same, although it will fall relatively slower and not as far, due to gravity not appearing the same effect at different scales.
button. (If the Hair shelf is not visible, see Load the default shelves in the Basics guide.)