<p>There are differences depending on particular schools, but here's a quick, general breakdown.</p>
<p>Under 5000 -- Actually, most LACs are 2,500 or so and under. So that's the first natural break. These schools exist for one reason: to educate undergraduates. That's their raison d'etre. They focus on this exclusively and, in my opinion, that focus tends to pay off in a better educational experience for most undergrads. Class sizes tend to be small. Students tend to know their professors. Professors do some research and they depend on undergrads to help them with it. When it comes time to get a recommendation or two for grad school, students are more likely to get something personal and meaningful from a professor who actually knows them.</p>
<p>The drawbacks are that the number of departments is limited, the number of classes is limited, and one cannot opt into grad courses because there are none.</p>
<p>2,500 to 5,000 -- Generally, these schools have some graduate departments, so the entire focus isn't on undergrad education. Still, schools of this size usually have small classes and the vast majority of faculty teach them. Research is often less available to undergrads.</p>
<p>On the other hand, schools of this size tend to have larger libraries than the LACs. They may have a few other extras, such as observatories, nuclear reactors, or what have you. There are some grad courses in some disciplines, and some undergrads can get into them.</p>
<p>5,000 to 20,000 -- This is a huge range. 20,000 is a fairly large school, while 5,000 is going to feel much more intimate. Basically, as you move up the numbers, classes (especially entry-level ones) get larger, the number of grad students gets larger, the class offerings get larger, and resources (assuming they are available to you) become more available. </p>
<p>Typically, as you get to 10,000 and above, you start to get the HUGE undergrad courses in things like psych, econ, chemistry, biology, etc. these courses can run 500 students or more. Personally, I believe you can learn as much from a DVD of the lecture as you can from actually attending. There are typically breakout and/or lab sections, and these are run by teaching assistants, who are grad students. Upper level courses tend to be smaller, but it's still common to have class sizes of 40+ which means that most of these things are pure lectures.</p>
<p>Mega universities -- Here you get famous faculty (in some cases) that you will probably never see. Many of these schools almost completely remove the teaching burden from top researchers so that they can focus on research and, more importantly, research grants. There are typically more courses, more student clubs, big time sports, more impersonality, etc. About what you'd expect.</p>
<p>These are stereotypes, only. There are exceptions. Many of them.</p>