extent vs length

extent and length both are nouns.

Word	Noun	Adjective	Verb	Adverb
extent	Yes	No	No	No
length	Yes	No	No	No

As nouns, length is a hyponym of extent; that is, length is a word with a more specific, narrower meaning than extent:

extent (noun)	length (noun)
the distance or area or volume over which something extends	a section of something that is long and narrow
the point or degree to which something extends	the linear extent in space from one end to the other; the longest dimension of something that is fixed in place
	continuance in time
	the property of being the extent of something from beginning to end
	size of the gap between two places