length vs arm's length

length and arm's length both are nouns.

Word	Noun	Adjective	Verb	Adverb
length	Yes	No	No	No
arm's length	Yes	No	No	No

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

Other hyponyms of length include leg, gauge, light time, skip distance, wingspan, wingspread, wingspread, altitude.

length (noun)	arm's length (noun)
a section of something that is long and narrow	a distance sufficient to exclude intimacy
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