How to modify the grid spacing on the main window?

LightBurn provides controls allowing you to set as you describe, yes. These controls live on the ‘Settings’ page displayed when you click the ‘Gears’ icon near the top-center of the Main Window. The control you want to edit is titled ‘Visual Grid Spacing’ (number 3 in image below).

Note in the following image, the ‘Visual Grid Spacing’ is set to 1.0000" with a 5" x 7" rectangle placed in the far upper-right of the workspace.