Had to laugh at this post -- NOT the poster, mind, but the topic -- because I have every size square from LITERALLY 2" up to 20+". And 4 of the 8" Bias Square Rulers from That Patchwork Place! (Sort of shows how much I value that ruler above all others, LOL!!)
I rarely use the 15" one, admittedly, and assume I only have it because it was leftover when I closed down my shop all those years ago. I use the 20" for square-up purposes almost exclusively. 8" and 12.5" are my go to squares.
Jan in VA