length vs lengthiness

length and lengthiness both are nouns.

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

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

length (noun)	lengthiness (noun)
a section of something that is long and narrow	amount or degree or range 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	the consequence of being lengthened in duration
continuance in time
the property of being the extent of something from beginning to end
size of the gap between two places