Class sizes at Catholic colleges will vary based on the size of the university and the popularity of the majors. Additionally, in the case of most universities, freshman classes will almost always have larger class sizes than major-specific and upper-level classes. That said, Catholic colleges typically have smaller class sizes than non-Catholic public or state universities.
The average class size for Catholic colleges is around 15-25 students. Some required freshman courses will have larger averages, and some more specific and less popular majors will have smaller averages. However, this is a general statistic. Many state or public universities will have much higher averages, some between 100-400 students per class, especially for required freshmen courses.
As the classes get more advanced and major-specific, the average does decrease, sometimes to 20-40 per class. Even then, the class sizes at state or public universities are still usually bigger than at Catholic colleges. However, compared to other private non-Catholic universities, average class sizes are very similar.
Even when comparing different Catholic Universities there will be differences in average class sizes between larger and smaller schools, which is to be expected. Generally, though, Catholic colleges will have smaller average class sizes than public or state universities, which allows for more a personalized learning experience than one can get in a classroom of over 100 students. To get a better idea of what sort of class sizes to expect at a university, search the website of the specific Catholic college(s) for the student to professor ratio, or ask an admissions officer. Typically, smaller ratios mean smaller average class size.