This is the scale using Magic_Als scale, but it seems like LC is WAAY to small. Can someone check me on this
Here is a map of Liberty City at 1 meter per px.
The map of Los Santos has about 77 px per 1,000 ft (= 304.8 m).
1,983 (map width in px) / 77 * 304.8 = 7,850 (map width in meters).
So what you see below is the correct overlay, at 5 meters per px.
Also see here for 1 square km in both games.