It largely means how you define "space", specifically how far about the earth's atmosphere you consider to be the start of space...
The problem with the Great Wall of China is that it's so long and thin. There are many images of the earth taken from the space shuttle where extremely large objects such as hurricanes are clearly visible, and the outlines of large cities can be seen during their nighttime because of their light pollution.
However, fire up Google Earth and you can see pretty much *anything* from "space" - I can currently see my car parked outside my house...