Auditory size perception refers to the ability to make accurate judgements of the size of a sound source based solely upon the sound emitted from the source. Electro-physiological and behavioural data were collected to test whether sound-source size parameters are detected from task-irrelevant sequences in adults and newborn infants. The mismatch negativity (MMN) obtained from adults indexed automatic detection of changes in size for voices, musical instruments and animal calls, regardless of whether the acoustic change indicated larger or smaller sources. Neonates detected changes in the size of a musical instrument. The data are consistent with the notion that auditory size-deviant detection in humans is an innate automatic process. This conclusion is compatible with the theory that the ability to assess the size of sound sources evolved because it provided selective advantage of being able to detect larger (more competent) suitors and larger (more dangerous) predators.