It's about thirty times as long as Hadrian's Wall
In other words, 623 leagues is 29 times the length of Hadrian's Wall, and the length of Hadrian's Wall is 0.03 times that amount.(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 22 leagues (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 thirty times as long as The 405
In other words, 623 leagues is 29.701 times the length of The 405, and the length of The 405 is 0.03367 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 20.9760 leagues. 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 thirty times as wide as The English Channel
In other words, 623 leagues is 32.3 times the width of The English Channel, and the width of The English Channel is 0.03 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 19.30 leagues, 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 forty times as long as The Panama Canal
In other words, 623 leagues is 39 times the length of The Panama Canal, and the length of The Panama Canal is 0.03 times that amount.(Canal de Panamá) (Panama)
Connecting the Atlantic and Pacific Oceans across the Isthmus of Panama, the Canal measures 16 leagues in length. Ships passing through the Canal are raised (and then lowered) 0.0047 leagues above sea level through the lock-and-dam system.
It's about one-one-hundredth as long as The Distance from Earth to the Moon
In other words, 623 leagues is 0.00900380 times the length of The Distance from Earth to the Moon, and the length of The Distance from Earth to the Moon is 111.064 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 69,193.03 leagues 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 150 times as tall as Olympus Mons
In other words, 623 leagues is 130 times the height of Olympus Mons, and the height of Olympus Mons is 0.008 times that amount.(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 4.90 leagues. 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 300 times as tall as Maxwell Montes
In other words, 623 leagues is 310 times the height of Maxwell Montes, and the height of Maxwell Montes is 0.003 times that amount.(Ishtar Terra, Venus)
Maxwell Montes rises to a total height of 2 leagues. 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.