All square rulers that I have seen have markings on them so you can use them to cut down to any size you want. If you have a 12 1/2" square and all the other sizes you list you can use them to cut down to any size you want, including fractions. It just requires you to flip your piece and cut twice.
I use my 6 1/2 to trim down to as small as 1 1/2 and everything in between. I use my 12 1/2" to square up and trim anything larger than 6 1/2" and I will use two rulers if I need anything bigger.