Depends on what you want to square up. If it is a block, get one for the size of block you make most -- I have them from 3.5 - 12.5 because I also use them to fussy cut blocks. If you are squaring up your top/quilt after quilting, then I would go with the 20.5 if you can afford it. That said, I used the 6x24 ruler as my square up ruler for many, many years.