The P1324 Half Size diffuser is named by its prime 13 quadratic theory sequence and it's size: 12 x 24 inches square.  The depth is 3.5 inches.  The units can be screwed to your walls, or placed into a frame to become free-standing panels.  These panels are sold in pairs only.  There is absolutely no point in buying a single panel, as by itself it would do almost nothing.  Diffusion works by cumulative repetition.  That means having some on all four walls exponentially improves the performance of the diffusers on each wall.