I have a 20" square ruler......while it is huge, I originally used it for squaring up T-shirts, it has come in quite handy and I'm not sorry I have it, however, I really don't need on that large....I also use my 12" and 10" more often......I also often use one that is 8.1/2" That seems go be a popular sized block and of course, any of these can be used for smaller blocks.