I eliminated the 1" black border all around the rail fence to come up with a block that's 13" x 15". The problem with this is that rail fence is designed as a square block. You can still do it as a rectangle, but the strip width is different on the horizontal and vertical rails. I've attached a JPG of the block at that size with rotary cutting measurements. I think if I was doing it I'd stick with a square rail fence at 13" x 13" and then add a one inch strip top and bottom (besides the black) to make it 13" x 15". Let me know if you want that measurement.