I think you may find it difficult to make a 12.5 (12" finished block). I would interpret it as 9 patch with sashing and a green cornerstone. If you look at it as a single block, it's a 7X7 patch which would be difficult to do as a 12 inch block without really strange measurements. would you settle for a 14" (finished) block? If so, the small squares would finish at 2".