As others are saying, that varies widely from school to school and even from department to department within a college or university. A broad generalization is that introductory courses are larger than more advanced courses ("lecture" vs. "seminar"), humanities courses tend to be smaller than courses in science departments, and "colleges" (which focus on teaching) are likely to have smaller classes than "universities" (which focus on research)--though this varies greatly. An introductory lecture course can have thousands of students (though most universities will break the class down into multiple smaller "discussion groups," usually led by graduate students, so that students have a chance to ask questions and get more focused instruction on key topics. On the other hand, I've taught seminars that had only four people in them (which is unusually small; the more usual seminar size is anywhere from 10-30 or so).
The questions you want to ask of a prospective college, if you want smaller classes (which most students do) are:
- How big is the typical introductory class in (whatever department/field you're most interested in)?
- How big are the general ed (i.e., non-major) courses?
- Are there seminars for first- and second-year students? Are those seminars open to majors only, or are there seminars for general ed courses?
Keep in mind, too, that undergraduate courses, especially for first- and second-year students, are increasingly being taught by non tenure-track faculty; research universities and community colleges, in particular, may rely on graduate teaching assistants, adjuncts, and temporary or part-time faculty to help them offer smaller classes. While these non-tenure-track faculty can be great instructors, there are some disadvantages, from the point of view of students, to having most smaller classes taught by adjuncts: temporary faculty may not be around to write recommendation letters later on, they may not have offices where you can find them if you need help, they may not be familiar with the resources and support services that the college offers. So it's worth adding an extra question to the list above:
- How many first- and second-year seminars or discussion-based courses are taught by tenured or tenure-track faculty?