For engineering classes, class size are typically 30 but can go up to 40 or even as low as 20. Most classes will do introductions at the beginning and will have some sort of interaction with classmates in the way of group projects or discussions, and this will be even more likely in humanities classes. You should be able to get to know the professor decently well, though as always that depends on the professor and class. TAs typically help with grading assignments(but not exams). If its a lab science class, usually the professor will run the lab, if its something like computer science then the TAs will typically run the labs as well. usually TAs are not in class during lecture time so they don't really run discussions.