long has 11 distinct synonyms spread across 3 of its 12 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. Its one recorded opposite is short.
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.
primarily temporal sense; being or indicating a relatively great or greater than average duration or passage of time or a duration as specified
“a long life”
primarily spatial sense; of relatively great or greater than average spatial extension or extension as specified
“a long road”
holding securities or commodities in expectation of a rise in prices
“is long on coffee”
(of speech sounds or syllables) of relatively long duration
“the English vowel sounds in `bate', `beat', `bite', `boat', `boot' are long”
planning prudently for the future
“large goals that required farsighted policies”
5 of the 12 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for long.
Synonyms, antonyms and the broader and narrower relations for long come from Princeton University’s WordNet. Full sources and licences.