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We investigate when infants exhibit knowledge of the familiar size of well‐known objects and whether this knowledge is affected by stimulus format, that is, whether the stimuli are presented as real objects or matched pictures. Infants (130 7‐ and 12‐month‐olds) saw everyday objects such as sippy cups and pacifiers in their familiar size and novel sizes (larger or smaller than the familiar size) placed pairwise within infants' reach. We used a preferential‐looking paradigm to investigate whether infants are able to discriminate familiar from novel sizes. Although, infants of both age groups looked longer toward real objects that were smaller or larger than the familiar size, there were no looking preferences for the pictures. These results suggest that although 7‐ and 12‐month‐olds demonstrate familiar size knowledge for real objects, this understanding does not generalize to pictorial representations of those objects. [ABSTRACT FROM AUTHOR]

• 12-month-olds successfully discriminate between familiar- and novel-sized everyday objects. • 7-month-olds successfully discriminate between physically larger and smaller objects. • Familiar size influences behaviors other than interactions that had been shown in prior research. • Knowledge of the familiar size of objects seems to be acquired later than of human faces or bodies. The current study aimed to examine the age at which infants exhibit knowledge of the familiar size of common everyday objects. A total of 65 7- and 12-month-old infants were presented with familiar-sized and novel-sized (i.e., larger or smaller than the familiar size) common everyday objects (i.e., pacifiers and sippy cups), which were placed out of their reach. Both 7- and 12-month-olds' first looks were more frequently directed toward physically larger objects irrespective of whether they were familiar- or novel-sized objects. This finding indicates that initial visual orientation is contingent on the magnitude of the absolute physical size of an object. However, when the entire duration of presentation of the objects (i.e., 10 s) was examined, 12-month-olds' mean looking durations were found to be longer for novel-sized objects than for familiar-sized objects. Thus, although infants in both age groups were able to discern the physical sizes of objects, only 12-month-olds could successfully discriminate between the familiar and novel sizes of everyday objects. Notably, 12-month-olds demonstrated knowledge of familiar size even though the test objects were out of their reach and, consequently, unamenable to manual exploration. [ABSTRACT FROM AUTHOR]