WordNet records no synonym and no antonym for ampleness. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 property of being more than sufficient; comfortable sufficiency
“the ampleness of her servings more than satisfied his hunger”
the property of impressive largeness in size
“he admired the ampleness of its proportions”
Synonyms, antonyms and the broader and narrower relations for ampleness come from Princeton University’s WordNet. Full sources and licences.