16.5 is my largest one.

My largest is a 16" square up ruler from Quilt In A Day! Haven't used it yet, though!

I have 12 1/2 and 161/2 and use both!

12.5" I use it for every size block. Bought a 6.5 and it's too small.

15 or 15-1/2, can't remember if it includes the quarter inch seam or not and I am too lazy to go downstairs to look. LOL
I have a friend who has that huge 24 inch and when I borrowed it once, it created my setting on point section quickly. She said it cost her an arm and a leg however. :(

My largest square ruler in a Omnigrip 20 1/2 inch square. I have used it more than I ever thought I would. It was a good purchase for me.

I use a 12 1/2 inch square ruler

I have a 12 1/2" but wish I had a 16 for the t shirt quilt I'm going to make!

I have a Quilters Rule 161/2 ruler that I don' use much but I really use my 12 1/2 one a lot

I have a 16 1/2- used all the time for large blocks and pillow tops :)