If your blocks are close to the right size, and I is important to you to have them exact you can block them into submission. Mark a square the correct size on a piece of freezer paper, lay it on the ironing board. Pin the block to the paper with lots of straight pins matching the marked size. You can use spray starch to help stretch the block. once pinned in place press carefully, using more starch if necessary until you have the size you want. let it cool before removing pins. To shrink pin the same way, mist heavily with water, hold the iron just above the block.