length has 2 distinct synonyms spread across 2 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the linear extent in space from one end to the other; the longest dimension of something that is fixed in place
“the length of the table was 5 feet”
the property of being the extent of something from beginning to end
“the editor limited the length of my article to 500 words”
a section of something that is long and narrow
“a length of timber”
Synonyms, antonyms and the broader and narrower relations for length come from Princeton University’s WordNet. Full sources and licences.