Research on diverging device conditions remains scarce, particularly in relation to how they affect online shoppers' product assessments. This research investigates the effects of online shoppers' device type on the accuracy of their product size evaluations and reveals, for the first time, the complex mechanisms behind these effects by examining the role of confidence about product size, product familiarity, and shopping motivations. The findings from four experimental studies show that using a PC (vs. a smartphone) to view products results in greater size‐estimation errors than do smartphones. Moderated‐mediation analyses suggest that viewing products on a PC (vs. a smartphone) leads to overconfidence about product sizes, which then results in greater size‐estimation errors, and this mediation effect is stronger for unfamiliar products. However, the mediation effect is reversed with utilitarian motivations—PC users become more accurate in estimating product sizes. Overall, the findings encourage online retailers to consider their customers' device type as one of the factors influencing size‐related returns; thus, optimizing their product showcases by carefully monitoring customers' device information, which is easily identifiable through websites, is important in improving the accuracy of product assessment.