Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - Game Master Main map (the one where you place pins) question

I would say: "The bigger, the better".
It of course depends on what you want to do, but a larger file (in dimension) would allow you to zoom in more in case you want some locations to be really close to eachother and still have a decent quality map.
I am using one that is "only" 1500x1500 pixels and it suits me just fine.