Background Scaler
Adds a button to the scene grid configuration tab. Zoom in on map and click the corders of a grid square on the image, then right click to apply the scale to the background.
Please note that this tool is only as accurate as your clicks.
This changes the size of the background image, not the grid size.
Set your preferred grid size before using this tool, though the scaling can be done again after grid size change.
This tool can be used as many times as it takes to get it perfect.