It's about one-three-hundred-thousandth as long as The Amazon River
In other words, 21 meters is 0.0000032620 times the length of The Amazon River, and the length of The Amazon River is 306,600 times that amount.(from Nevado Mismi, Arequipa, Peru through near Marajó, Brazil) (approximate distance)
The Amazon River has an approximate distance of 6,437,000 meters. Although by most measures it is the largest and one of the most significant rivers in the world, the region surrounding the Amazon is mostly sparsely populated and the river itself has no bridges across it at any point.
It's about one-four-hundred-thousandth as long as The Great Wall of China (total)
In other words, 21 meters is 0.000002370 times the length of The Great Wall of China (total), and the length of The Great Wall of China (total) is 422,000 times that amount.(长城, 長城, Chángchéng, a.k.a. 万里长城, 萬里長城, Wànlǐ Chángchéng) (a.k.a. "The long wall of 10,000 Li") (from Shanhaiguan through Lop Nur, China) (total length, all branches)
The Great Wall of China, including all branches and trenches, is 8,850,000 meters. Built and maintained in multiple sections over about eleven centuries, the Great Wall is currently suffering the effects of erosion, especially in the older sections made primarily out of mud.
It's about one-six-hundred-thousandth as long as The Diameter of Earth
In other words, 21 meters is 0.00000164624880 times the length of The Diameter of Earth, and the length of The Diameter of Earth is 607,441.60 times that amount.(Equatorial)
The Earth Ã¢â‚¬â€ not a perfect sphere, but rather an oblate spheroid with bulged middle Ã¢â‚¬â€ has a diameter of approximately 12,756,273 m at the Equator. The first complete view of Earth's diameter was in a photograph taken from a V-2 rocket launched in 1946 by the United States Army, which reached an altitude of 104,607.360 meters.
It's about 0.0000005 times as long as The Circumfrence of Earth
In other words, 21 meters is 0.000000524017280 times the length of The Circumfrence of Earth, and the length of The Circumfrence of Earth is 1,908,334 times that amount.(Equatorial)
The Earth — not a perfect sphere, but rather an oblate spheroid with bulged middle — has an Equatorial circumference of approximately 40,075,014 m. The first airplane trip around the circumference of the Earth (with a few stops along the way), was completed in 1924 by the United States Army Air Service, and took 175 days.
It's about 3,000,000 times as long as a Red Blood Cell
In other words, 21 meters is 2,800,000 times the length of a Red Blood Cell, and the length of a Red Blood Cell is 0.000000360 times that amount.(a.k.a. RBCs, a.k.a. haematids, a.k.a. erythrocyte, a.k.a. erythroid cells, a.k.a. red blood corpuscles)
An average Red Blood Cell measures 7.5 µm (micrometers) in diameter. An average human has 20 - 30 trillion red blood cells in their body and each cell completes a circulatory lap in about 20 seconds.
It's about 3,500,000 times as long as a Cell's Nucleus
In other words, the length of a Cell's Nucleus is 0.000000290 times 21 meters.(for animal cell)
The nucleus of an animal cell measures an average of 0.00000600 meters. The nucleus is the largest organelle in these cells, accounting for about 10% of the cell's total volume.
It's about 10,000,000 times as long as a Bacterium
In other words, 21 meters is 11,000,000 times the length of a Bacterium, and the length of a Bacterium is 0.0000000910 times that amount.(Escherichia coli) (rod-shaped configuration)
An rod-shaped E. coli bacterium measures about 0.00000200 meters in length. A staple of biology labs, E. coli can reproduce in as little as twenty minutes.