How large are 1st, 2nd and 3rd year lectures/classes typically in Poli Sci, Econ or IR?

How big (how many students roughly) are in intro classes/lectures for subjects such as econ, poli sci, and international relations? Does every class have an TA discussion class that's separate? Do they stay as big in 2nd or 3rd year? Are there any classes in these subjects that are 25-30-ish seminar style? Or are they all really big lecture format 100-1000-ish?