It's about one-one-hundred-fifty-thousandth as long as The Volga
In other words, 21 meters is 0.00000595 times the length of The Volga, and the length of The Volga is 168,000 times that amount.(a.k.a. Волга, a.k.a. İdel (Идел), a.k.a. Атăл (Atăl), a.k.a. Edil, a.k.a. İdil, a.k.a. Рав (Rav)) (from the Valdai Hills, Tver Oblast, Russia through the Caspian Sea)
The largest river in Europe, the Volga river runs 3,530,000 meters in total length. World War II's Battle of Stalingrad between the Nazi Germany and the Soviet Union, which resulted in nearly two million casualties, was fought in part for control of the river.
It's about one-two-hundred-thousandth as long as The Mississippi River
In other words, 21 meters is 0.000005576 times the length of The Mississippi River, and the length of The Mississippi River is 179,300 times that amount.(from Lake Itasca, Minnesota through Pilottown, Plaquemines Parish, Louisiana)
The total length of the Mississippi River is 3,766,000 meters. Prior to the development of soil erosion programs, the Mississippi River would carry approximately 400 million metric tons of sediment from its banks to the Gulf of Mexico every year.
It's about 200,000 times as tall as a sheet of Paper
In other words, 21 meters is 200,000 times the height of a sheet of Paper, and the height of a sheet of Paper is 0.0000050 times that amount.(for US Letter, a.k.a. ANSI A; 215.9mm by 279.4mm (8.5 in x 11 in); 20lb)
A single sheet of 20 lb, 8.5 inch x 11 inch paper is just 0.0001 meters thick. This style of paper weighs a mere 5 g per sheet.
It's about 200,000 times as wide as a Strand of Hair
In other words, 21 meters is 210,000 times the width of a Strand of Hair, and the width of a Strand of Hair is 0.0000048 times that amount.(average)
A strand of human hair averages 99 µm (micrometers), with blond hair being the finest and black hair the thickest. On a healthy scalp, each strand of hair will last up to 6 years.
It's about one-two-hundred-thousandth as long as The Distance from L.A. to New York
In other words, 21 meters is 0.00000468 times the length of The Distance from L.A. to New York, and the length of The Distance from L.A. to New York is 214,000 times that amount.(Los Angeles, California to New York City, New York) (via I-10 E, I-15 N, I-70 E, I-76 E, I-80 E, I-280 E)
The total length of the freeway route from Los Angeles, California to New York City, New York via Interstate 80 is 4,490,000 meters long. This route intersects eleven states and several major cities, including Las Vegas, Nevada; Denver, Colorado; Lincoln, Nebraska; Omaha, Nebraska; Des Moines, Iowa; South Bend, Indiana; and Cleveland, Ohio.
It's about one-two-hundred-thousandth as long as I-80
In other words, 21 meters is 0.00000452778 times the length of I-80, and the length of I-80 is 220,859 times that amount.(a.k.a. Interstate 80, a.k.a. Keystone Shortway) (net length)
Interstate 80, crosses the continent of North America from Teaneck, New Jersey to San Francisco, California and covers a total length of 4,638,030 meters. A 60,000 meters stretch between mile marker 318 in Grand Island, Nebraska and mile marker 390 in Lincoln, Nebraska is the longest straight path in the US Interstate system.
It's about one-three-hundred-thousandth as long as The Great Wall of China (wall only)
In other words, 21 meters is 0.000003355 times the length of The Great Wall of China (wall only), and the length of The Great Wall of China (wall only) is 298,100 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) (wall segments only)
The Great Wall of China has total wall segment length of 6,259,000 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.