How large are the classes?
During freshman year, students are split into groups for common curriculum courses so each seminar has no more than around 24 students. The size allows students to get one-on-one interaction with the professor and ask them questions both during lessons and after class. It also means that you can schedule office hours with a professor for a consultation, even when deadlines are approaching. Common curriculum lectures, on the other hand, will contain the entire freshman cohort. For AY2017/18, that meant 230 students in the Class of 2020. As a result, all common curriculum lectures are held in the Performance Hall. If you want, it is still possible to ask lecturers questions after the lecture is over, purely because most students don’t normally stay behind to do things like that.
As for electives, class size may differ depending on the amount of students subscribed to a particular course. In general, though, the school attempts to maintain a small ratio between students and professors to really ensure high quality interactions between students and professor. Moreover, it guarantees an involved class discussion, because smaller sizes ensure that each student gets a chance to contribute intellectually to the topic at hand. Even better, many professors will split the class up into groups, so you are essentially having a discussion with two or three other people rather than sharing airtime with an entire class. If a course is particularly popular (which is especially true for introductory courses) it is likely to have a large class (also around 24 students), but the lectures will also be limited to only this small group. For these modules, lectures are held in the smaller Lecture Theaters.
Many modules at the higher level have small classes as different students will choose to specialize in different areas. Furthermore, modules that are only available to juniors and/or seniors will have less students in them since no underclassmen will be attending. It is not unheard of for a course to have a handful of students, but if a module is highly undersubscribed, it may be canceled altogether.
If a student decides to take a module at NUS, of course, it is unlikely that they will experience Yale-NUS class sizes. Many NUS modules are lecture-based and have very large class sizes.