hard to be sure without knowing what pattern you're going for, but it sounds like you'll have to cut all the panels to the same size...11 1/2 inches...or, add a sashing to the smaller ones to take them backup to the 12.5. If you add the sashing, you won't have to worry about math on the other blocks at all. The easiest way to do it ( but perhaps wasting a little fabric, is to simply cut a strip of coordinating fabric at least one and a half inch wide and stitch it to all four sides...once stitched and ironed, cut out your larger square to match the other panels.