Find a convenient male adult of roughly average size. Measure the distance from his shoulder to his wrist. You should find that it is about 22–23 inches (56–58 cm). That’s one of the oldest ways to define an ell, once the usual measure in large parts of Europe for textiles such as woollen cloth. It was considered to be roughly equal to six hand-breadths — a hand was 4 inches, a unit still used for measuring the heights of horses, which would make an ell about 24 inches.
Read further: in this article from World Wide Words: Investigating the English Language Across the Globe