A lighting fixture includes a mounting rail slidably attached to a first side of a main section. A lighting assembly includes a first LED board attached to the main section, and a second LED board attached to an extension movable longitudinally relative to the main section. The extension may be attached to the mounting rail and be slidable longitudinally relative to the main section, to extend the length of the lighting fixture. The second LED board may be supported on a slider extrusion rigidly attached to a spacer on the main section. The lighting fixture can be more quickly and easily installed with minimal field cutting of components.