A rectangular two-story horse barn is being designed for a farm. The upper floor will be used for storing hay, and the lower floor will have horse stalls that extend 5 feet from both of the longer walls. The barn's length is twice the barn's width, and the lower floor's ceiling height is 6 feet less than the barn's width. What should the dimensions of the lower floor be if the space not used for stalls is to have a volume of 1,920 cubic feet?