•We examine consumer size perceptions based on online product pictures.•Online size perceptions require more effort and are less accurate.•Size perceptions take more time and consumers pay more attention to labels online.•Consumers overestimate the sizes of small products online.•Consumers underestimate the sizes of large products online. This study examines consumers’ liquid volume size perceptions based on standardized product pictures in online stores. Four empirical studies compare liquid volume size perceptions for the same products when displayed online and offline. The findings indicate that size perceptions require more effort online than they do offline and that the online display decreases the accuracy of the size perceptions. Online size perceptions require more effort in terms of time and visual attention, as consumers need to study product labels to determine product sizes. The study also introduces a new visual bias to consumer research: the familiar size bias. A consideration of this bias reveals that consumer size perceptions are less accurate online, where consumers tend to overestimate the sizes of small products and underestimate the sizes of large products.