middling 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. 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.
any commodity of intermediate quality or size (especially when coarse particles of ground wheat are mixed with bran)
to certain extent or degree
“pretty big”
Synonyms, antonyms and the broader and narrower relations for middling come from Princeton University’s WordNet. Full sources and licences.