Is there a way I can calculate the size of my grid boxes on a graph in a GUI?

I made a GUI in witch I included zoom in and zoom out features. I need to calculate the size of each grid box each time the user zooms in and out. This is because I need to calculate how many solar panels I can fit in any given box. I already know how to calculate how many solar panels can fit in a grid box with fixed dimensions. But I am struggling to figure out how to do this with changing grid dimensions.

Any help would be appreciated.

Sincerely, struggling student