I also mostly use my 12.5" square ruler for squaring up unless it's a small block--then I use my 6.5" square.
Find whatever is supposed to be the center and lay your ruler down so that one of the measurement marks is on a seam line if possible and the center is dead on where you want it then trim anything that hangs over on your right and top (left and top if you're left handed). Spin the block 180 degrees and lay the ruler down on the exact same center and seam lines and trim the excess again. I have a small revolving cutting mat that makes spinning the blocks easy but I usually do my squaring at my regular cutting table and just pick them up and turn them manually.
Now, if your problem is that your blocks are too small instead of too large; try re-pressing the seams first. If they're still too small--you'll need to block them instead of squaring them or add a sashing to each side and square to size. Blocking will only work if your block is less than 1/4" or so off. Any more than that and you'll pop seams trying to stretch it.
Long story short--It depends on the block and what you are trying to do with it.