FWIW, I can use wmctrl to resize the LightBurn window to a useable size after starting it using the command:
wmctrl -r "<untitled>" -e 0,87,30,1261,720
There might be a ‘tighter’ linkage to the app using something other than "<untitled>" but it’s too chilly in the loft right now to mess with it.
I suppose this could be a temporary workaround, but I find it odd that the LightBurn window always starts up larger than it should. FYI, wmctrl -lG listed the vertical size of the freshly loaded window as 851 pixels. Physical screen size is 768.
UPDATE: I put a launcher for my script on the Desktop and that will work fine as a workaround for the time being.