terrain.party, best scale size for realistic maps?

Since the map should be the same to scale anywhere you go, shouldn't there only be one scale size(default is 18km) no matter where you go? I'm wanting to use it to make a height map but not sure which one I should use to make the most accurate representation of the area I'm trying to recreate. Why would I want to change the scale? Wouldn't that make the height map not in proportion if I change it too much?