How thick is the Great Wall of China because it can be seen from space so it must be wide?

It's a myth that the Great Wall of China can be seen from the moon.

http://www.snopes.com/science/greatwal.htm

Upon viewing the Great Wall on Google Earth, it is clear that past an elevation of 600 miles the ability to see any of the wall diminishes.

As for "seen from space" most certainly. Astronauts leave Earths atmosphere and enter space at 350 miles.

[Link no longer works]liftoff.msfc.nasa.gov/academy/space/atmosphere.html

This site gives average values to height and width.

https://encyclopedia2.thefreedictionary.com/Wall+of+China

The wall's present form dates substantially from the Ming dynasty (1368–1644). It averages 25 ft (7.6 m) in height and is 15 to 30 ft (4.6–9.1 m) thick at the base, sloping to 12 ft (3.7 m) at the top.

It's of the order of 20 feet wide, so a lot narrower than most double carriage roads anywhere. In addition it winds over and round hills, so is highly unlikely to be seen at all from 'space', let alone to be the 'only man-made object' that can be so seen, as per the myth.

One must take into consideration that the wall is thicker at the bottom. The surrounding area next to the wall (how many linear feet, I do not know) are also tell-tale markers for the width of the wall.

Overall the footprint of the wall must be 100 - 150 wide, or more. I would say that if one knows what one is looking for from "space", the path of the wall is no different than say a large river.

Here is a NASA photo page. Have fun!

http://www.nasa.gov/vision/space/workinginspace/great_wall.html

Whether the wall really can be seen from space is immaterial to the question. Here's another site indicating that the portions most frequently visited by tourists are 15-30 feet in height, 25 feet wide at the base, and 15 feet wide at the top, not counting the bastions:

http://www.logoi.com/notes/great_wall_of_china/the_great_wall_of_china_3.html