I learned this from a quilt guild member. If your blocks are slightly too small, you can iron them a little larger.
Take a piece of heavy muslin about 15 inches square. On it draw squares with a permanent marker starting about 5 or 6 inches. Go out 1/4 inch on all sides until you get a square about 12 1/2 inches. I keep this under my cutting board.
Now when you want to square up or enlarge a block, simply lay it evenly on the muslin, and, using a STEAM iron, iron it until it fits perfectly on the size square you need.
I hope this makes sense. I have a tendency to sew a little larger than 1/4 inch seam, so my blocks have a tendency to be a little too small.