http://store.quilting-warehouse.com/...-omnigrid.html
Here is a great assortment of different sized rulers. You do not have to buy one of each size. I have one that is a 12 1/2" x 12 1/2" to make a finished size block of 12". I also have smaller ones and rectangular ones. I can use almost any size to square up a block, but it is best to use one that is larger that you need rather than smaller. As long as the corners have 90 degree angles, you will be fine. :-)
I should have asked; when you say ruler are you talking about a school room ruler, or an actual acrylic rule?