Maybe this is standard across mapping engines, but in MapGuide if I set the scale to 7 million I can see about half the world. At about 4 million I can see a continent. What unit of measurement could this scale possibly be in?
As whuber wrote in his comment: Scale is unitless. It expresses a ratio of a distance on the map to a distance on the earth.
e.g. A scale of 1:150,000 means that 1 cm on the map equals 150,000 cm or 1.5 km in the real world.
The convention is 72 pixels to the inch. I couldn't say for sure what MapGuide is using.