Class size is a first-order consideration in the study of education cost and effectiveness. Yet little is known about the effects of class size on student outcomes in online college classes, even though online courses have become commonplace in many institutions of higher education. We study a field experiment in which college students were quasi-randomly assigned to either regular sized classes or slightly larger classes. Regular classes had, on average, 31 students and treatment classes were, on average, ten percent larger. The experiment was conducted at DeVry University, one of the nation’s largest for-profit postsecondary institutions, and included over 100,000 student course enrollments in nearly 4,000 classes across 111 different undergraduate and graduate courses. We examine class size effects on student success in the course and subsequent persistence in college. We find little evidence of effects on average or for a range of course types. Given the large sample, our estimates are precise, suggesting that small class size changes have little impact in online settings.