Class size is a first-order consideration in the study of education production and education costs. How larger or smaller classes affect student outcomes is especially relevant to the growth and design of online classes. We study a field experiment in which college students were quasi-randomly assigned to either a large or a small class. All classes were conducted online. Large classes had, on average, ten percent more students than small classes (mean increase 10.2 percent, standard deviation 4.0 percent). The experiment was conducted at DeVry University, one of the nation's largest for-profit post-secondary 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 on student persistence in college. We find little evidence of effects on average or for a range of specific course types. Given the large sample, our estimates are quite precise, eliminating the likelihood of any meaningful effect of class size changes of the magnitude in question.