<p>Uh… back to answer the class sizes question, many lecture classes are fairly large with upwards of 150 people; Human Behavioral Biology (which is awesome) had 600something students. It is possible to get class participation in lectures, but not so much in chem/bio.</p>
<p>In this case, office hours are a great time to talk to the professor. Sections (with maybe 15-20 people) are also great for discussing with your peers.</p>
<p>In many departments, upper division classes tend to be smaller, and you will also encounter seminar-style classes. I highly advise taking an introductory seminar to get the experience of working more closely with the professor and classmates.</p>