The length of the lower arm is determined by the mounts. Make them first. Then take the springs off the axle and jack it up into fully stuffed position and measure. That will be your length. Most people start with the arm, or try to when building a long arm. But in reality, you need to build the mount first, and decide on whether or not your cross member will be removable and what type of bushings or rod ends you will use. Then build the arm to fit those specs. That is the easiest part.