Length

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Old English lengthu, of Germanic origin; related to Dutch lengte, also to long1.

wiktionary

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From Middle English lengthe, from Old English lengþu(“longness; length”), from Proto-West Germanic *langiþu, from Proto-Germanic *langiþō, equivalent to long +‎ -th. Cognate with Scots lenth, lainth(“length”), Saterland Frisian Loangte(“length”), West Frisian lingte, langte(“length”), Dutch lengte(“length”), German Low German Längde, Längd, Längte, Längt(“length”), Danish længde(“length”), Swedish längd(“length”), Icelandic lengd(“length”).

etymonline

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length (n.)

Old English lengðu "property of being long or extended in one direction; distance along a line," from Proto-Germanic *langitho, abstract noun from *langaz "long" (root of Old English lang; see long (adj.)) + *-itho, abstract noun suffix (see -th (2)). Cognate with Old Norse lengd, Old Frisian lengethe, Dutch lengte.

Figurative sense of "the distance one goes, extremity to which something is carried" is from 1690s. Phrase at length "to full extent" is attested from c. 1500. As "the length of a swimming pool," 1903. From the notion of "a piece or portion of the extent of anything" come the theater slang sense "a 42-line portion of an actor's part" (1736) and the sporting sense "the length of a horse, car, etc. in a race" used as a unit of measure (1650s).