The great wall of china is 4,6 to 9,1 meters wide at the base, and at the base it is 2,7 to 3,7 meters.
See the related link.
meters
Multiply the height by the width Area = length x width or (height x width in this case)
If it's rectangular - which is usually the case for a wall - you simply multiply length x width.
The width of the Great Wall is 5 to 10 meters (16 to 32 ft) at its base and generally 5 meters (16 ft) at the top. Various guard towers and gates form wider sections. The wall was from 6 to 9 meters (20 to 30 ft) tall and most estimates place its total length at more than 6400 kilometers (4000 miles).
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Great Wall of China
12.86 meters
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The width of the Great Wall of China varies from 15-feet to 30-feet at different locations along its length.
Although the extent of the Great Wall is some 6400 kilometers (4000 miles), it is only a maximum of 30 meters (95 feet) in width at its gates, and the material of the wall is similar to the surrounding land. So it would be far too narrow to observe from the Moon. (Even from low Earth orbit, astronauts need binoculars to make it out.)
Great Wall of China
about 3 meters