length vs extent

length and extent both are nouns.

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

As nouns, extent is a hypernym of length; that is, extent is a word with a broader meaning than length:

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