The diagrams have variable height to width ratios. I have seen examples ranging from square to 3:1. The thing that determines the ratio in this generator is the number of rows and columns.
e.g. On an 8.5 x 11 document, a 6 x 6 grid will give you slightly taller-than-wide diagrams. If you want them more square, decrease the number of columns or increase the number of rows (or both).* See how it works? You'll get the hang of it. :-)
* You could also change the document dimensions to something square.