Diet analysis of larval fish may clarify the role that zooplankton—not only its abundance but also its size structure and taxonomic composition—may have on larval growth and subsequent recruitment levels. Taking into account the increase in size during larval stages, we followed a larval size-dependent approach for the analysis of prey size in the diet of larval sardine (Sardina pilchardus). Studies on feeding patterns of clupeid larvae typically involve a problem due to regurgitation and defecation of gut contents during capture. Therefore, an alternative sampling method was tested in this study, but no significant differences from conventional methods were found. Despite the low feeding incidence observed (23%), we found a circadian feeding pattern with the highest mean gut contents after dawn, decreasing during the day and with the lowest values at night. Diet was mostly composed of copepod developmental stages, mainly nauplii, and prey size increased with larval size following a power function. Maximum and mean prey size were related to larval mouth gape, though other factors may be restricting the maximum prey size ingested, since most prey width values were between 20 and 40% of larval mandible width.