I use my 6 1/2 X 24" and my 12 1/2" square rulers the most. I use a 6 1/2" square that I use for smaller pieces.
I have all sizes of rulers. I'm a gadget person. I buy lots of rulers and gadgets.
I have a 20 1/2" square ruler that I seldom use but, when I need it, I'm glad I have it.