It's about 55 times as long as Hadrian's Wall
In other words, the length of Hadrian's Wall is 0.019 times 6,400 kilometers.(a.k.a. Roman Wall, a.k.a. The Wall, a.k.a. Vallo di Adriano, a.k.a. Vallum Aelium, a.k.a. Severus' Wall) (from North Tyneside, Tyne and Wear, England, United Kingdom to Cumbria, North West England, United Kingdom)
Hadrian's Wall, which crossed England form the North Sea to the Irish Sea during the time of Roman rule, measures 120 km (about 79 Roman miles). Roman soldiers — at one time up to 10,000 of them — only occupied Great Britain for about 246 years after the wall was built, after which time the local Brits began to take residence the wall's garrisons.
It's about 55 times as long as The 405
In other words, 6,400 kilometers is 54.9170 times the length of The 405, and the length of The 405 is 0.018209 times that amount.(a.k.a. I-405, Interstate 405, San DIego Freeway) (Southern California)
Running from Irvine to San Fernando, The 405 runs a total of 116.541 km. The 405 is the most heavily-trafficed freeway in the United States, with an average speed as low as 8 kph during rush hours.
It's about 60 times as long as The English Channel
In other words, 6,400 kilometers is 59.80 times the length of The English Channel, and the length of The English Channel is 0.0167 times that amount.(average width between Isle of Ushant and The Walde Lighthouse) (La Manche, Ärmelkanal, Mor Breizh, Mor Bretannek) (a.k.a. The Channel)
The English Channel measures between 180 km to 34 km, narrowing as it flows northeast. The world speed record for a Channel crossing by swimmer was set by Australian Trent Grimsey in 2012, with a time of 6 hours, 55 minutes.
It's about one-sixtieth as long as The Distance from Earth to the Moon
In other words, 6,400 kilometers is 0.01664774 times the length of The Distance from Earth to the Moon, and the length of The Distance from Earth to the Moon is 60.06821 times that amount.(a.k.a. Luna) (median of semi-major axis, or apigee, and semi-minor axes, or perigee)
The orbit of the Moon around the Earth is elliptical, with an average distance of 384,436.5 km from center-to-center. About 428.376 kg of moon rock have been cataloged on the Earth, from the American Apollo and Soviet Luna Programs, and from samples that have been extracted from meteorites.
It's about 75 times as long as The Panama Canal
In other words, the length of The Panama Canal is 0.014 times 6,400 kilometers.(Canal de Panamá) (Panama)
Connecting the Atlantic and Pacific Oceans across the Isthmus of Panama, the Canal measures 88 kilometers in length. Ships passing through the Canal are raised (and then lowered) 0.026 kilometers above sea level through the lock-and-dam system.
It's about 250 times as tall as Olympus Mons
In other words, the height of Olympus Mons is 0.0042 times 6,400 kilometers.(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 27 km. The mountain has been known to astronomers since the nineteenth century because it is tall enough to rise above Mars' frequent dust storms.
It's about 600 times as tall as Maxwell Montes
In other words, 6,400 kilometers is 580 times the height of Maxwell Montes, and the height of Maxwell Montes is 0.0017 times that amount.(Ishtar Terra, Venus)
Maxwell Montes rises to a total height of 11 km. Since Venus does not seem to have the kind of tectonic activity which gives rise to mountains on Earth, the origin of the Venusian mountain remains the subject of some dispute.