Both the horizontal and vertical sashing should be cut the same size as the block, 12.5 inches. The width is up to you. I would lay the uncut sashing fabric on a table or board, then arrange some of the blocks to see what width you like with your blocks. There's no set rule. What ever width you decide on, add 1/2" for the seam allowance. For instance, if you think 2" sashing will look good, cut the strips 12.5 X 2.5. The cornerstones would be a square cut to the width, 2.5C2.5