There's no single average class size for Ohio State University. The average class size varies dramatically depending on the course, department, level (undergraduate vs. graduate), and even the instructor. Some classes might have just a handful of students, while large introductory lectures can easily have hundreds.
To get a more precise idea, you'd need to look at specific department data or individual course listings. Ohio State's official website might provide some aggregate data on average class size, but it likely won't be a single, universally applicable number.