extensive has 10 distinct synonyms spread across 2 of its 3 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 intensive.
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.
large in spatial extent or range or scope or quantity
“an extensive Roman settlement in northwest England”
broad in scope or content
“across-the-board pay increases”
of agriculture; increasing productivity by using large areas with minimal outlay and labor
“producing wheat under extensive conditions”
Synonyms, antonyms and the broader and narrower relations for extensive come from Princeton University’s WordNet. Full sources and licences.