"Grid size" refers to the measurement or dimensions of a grid, which is a network of intersecting lines forming squares or rectangles. It indicates the number of rows and columns within a grid, determining its overall size and shape. Full definition
If they were, they would have given better answers as they got larger, faster, and used smaller grid sizes... but they continue to give the same answers. (judithcurry.com)
With it, you can change and customize home screen icons or set a custom grid size for a unique home screen layout. (howtogeek.com)
That's also where you'll find options to use a custom icon pack as well as change the home screen grid size, which I find to be extremely helpful. (9to5google.com)