For window panes?
I think it started out as a cylinder, as for say a glass bottle made by a glass blower.
While still hot, it's placed on a cooling table with an engraved template, for wont of a better description.
The ends are cut to the template, and the cylinder split and laid out, then also cut.
If you look at old windows, you'll see that they consist of small panes. I'm guess that pane size was deemed the most efficient for production for glass blowers at the time.
Some people claim that stained glass windows in old churches are thicker at the bottom than at the top because glass flows slowly like a liquid. We’ve known this isn’t true for quite some time now; these windows are thicker at the bottom owing to the production process. Back during medieval times, a lump of molten glass was rolled, expanded, and flattened before being spun into a disc and cut into panes. These sheets were thicker around the edges and installed such that the heavier side was at the bottom.
https://io9.gizmodo.com/the-glass-is-a-liquid-myth-has-finally-been-destroyed-496190894
The Corning Museum of Glass
https://www.cmog.org/article/does-glass-flow
More details on old window glass production.