The Extra Large Kneebak Pillow measures 18″ wide and has a wedge-shape design as well.  Placing it behind your knees to lift both legs will immediately stretch your low back muscles.  By flexing your legs one at a time, up and down, you not only stretch your back muscles but also allow for isometric movement that increases blood flow to your legs.
If using the Extra Large Kneebak in your recliner, be sure to place the closure side towards your back to support your low back and fill the gap at the base of the chair.  This is an area where most chairs fail in supporting the low back.