In the traditional brick-and-mortar classroom, small classes are generally perceived as desirable, but the benefits associated with particular class sizes in online education have not yet received much scholarly attention. Using a dataset of 10,648 enrollment records generated during the 2013-2014 school year at a state virtual school in the Midwestern U.S., this study examined the relationship between class size and student learning outcomes. The results of hierarchical linear modeling with fractional polynomial analysis suggest a reverse-U-shaped relationship, in which increasing online class sizes had a positive impact on achievement until the number of students reached 45, but a negative one if numbers increased beyond that level. At the subject level, similar reverse-U-shaped patterns were observed in math, social science, and other subjects, but not in English, foreign languages, or science.