The length of lights needed for a Christmas tree depends on the size of the tree. A small tree may only require a few feet of lights, while a large tree could require several hundred feet of lights. The best way to determine how much light you'll need is to measure the circumference of the tree and then estimate the number of lights that will fit along the perimeter. It's also important to make sure the lights are evenly spaced so the tree looks well-lit and not too sparse.