In response to a charge from the University Senate, a task force examined the influence of class size on student success in first-year composition courses that satisfy general education composition requirements. Using pre-existing data on first-time, first-year students with the entry cohort ranging from 2004 to 2015, a random forest analysis conducted on a sample of 22,147 students revealed class-size thresholds of 24-25 students in entry-level writing courses and 21-23 students in intermediate writing courses as the primary cut point for predicting success on a university designed writing proficiency exam. Implications for pedagogy, assessment, and student success outcomes are discussed.