WordNet records no synonym and no antonym for size constancy. It is the only member of its synset, 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 tendency to perceive the veridical size of a familiar object despite differences in their distance (and consequent differences in the size of the pattern projected on the retina of the eye)
Synonyms, antonyms and the broader and narrower relations for size constancy come from Princeton University’s WordNet. Full sources and licences.