These data represent sizes of individuals of the sea urchin species Strongylocentrotus franciscanus (also known as Mesocentrotus franciscanus) and Strongylocentrotus purpuratus on permanent subtidal monitoring sites around San Nicolas Island between 2014 and 2019. The sites usually have been monitored twice per year, most often in October and April. Divers measured in situ urchin test diameters in millimeters. Approximately 200 individuals of each of the two species was measured at each supersite (pair of adjacent sites) during a sampling period.