There are two basic ways to define the size of your truck nerf bar. You can measure the length of your cab because the length of the nerf bard is supposed to equal it. The beginning of the nerf bard coincides with the front wheel ends, whereas the end of the nerf bard coincides with the rear wheel. Next, you should measure the width or diameter. Here, it is always individual and depends on your vehicle’s model year. The diameter of nerf bars commonly varies between 2 and 6 inches, so yours should fall within this range as well.