This paper examines the effect of class size on student's academic and behavioral performance in synchronous online courses, utilizing student-level administrative data and website clickstream data from a research university in China. By examining variations in class sizes within students but across classes, we revealed a significant negative effect of class size on students' course grades and satisfaction. Notably, reduced class interaction opportunities, rather than class attendance, emerged as the most robust channel through which these negative effects manifest. Our analysis also examined the non-linear impacts of class size, revealing pronounced negative effects on course grades in larger classes, particularly those with very high student counts. The decline in course satisfaction was most pronounced when class sizes increased from 2-15 to 16-24 students. Additionally, there was a noticeable decrease in class attendance as class sizes increased from 16-24 to 25-55 students and a further decline when class sizes increased from 25-55 to 56 or more students. The study also found that larger class sizes, especially those with very high student counts, were associated with reduced class interaction.