Preliminary stats show 55 students in my algorithms course. That's probably close to the mean and slightly above the median. It's certainly not the largest course in the School of Engineering and Applied Sciences (SEAS), but it's up toward the top.
Why should I, or any faculty member for that matter, care about class size? For junior faculty, at least, there's a clear answer. A tenure case without some teaching of significantly sized classes is one with a weakness, opening the way to the arguments that the faculty member in question is working in an area of little interest (since nobody wants to take their classes in their area) or is providing insufficient service to the department (since they haven't taken on a large core course).
For senior faculty, I'm not so clear. While class sizes are listed in our annual review, I've heard no mention that they're considered of any particular importance -- or even of non-zero importance -- in determining annual raises. Indeed, I can't think of any direct benefit to me personally for teaching a large undergraduate class as opposed to a small one.* One might want to take on a big class to support one's department, as ostensibly money (and positions) should, in some way, follow students at the departmental level. I'd like to think that's how it works at many places; however, recent conversations with some higher-ups suggest that that connection is fairly tenuous for SEAS. Harvard's system in that respect seems to be broken. (If it wasn't, I think we'd be further ahead in our hiring in CS.)
So why should I care about my class size? Primarily, I suppose, personal pride. I take satisfaction in teaching students; the more qualified students, the better. (Not the more students the better, though; the more qualified students, the better...) Indeed, I've done the math, and while I'm quite sure Harvard does not calculate things this way, in my mathematical model I'm earning what Harvard's paying based solely on the number of students I teach. That helps me sleep at night.
Overall, however, this seems like an area where the incentive structure doesn't seem set up right. I can understand that class size isn't an end in itself; indeed, I can understand that part of the mission of the University is to preserve knowledge in areas that might be of narrow interest. (The Sanskrit, Slavic, Turkish, and Yiddish courses, for instance, have remarkably low numbers.) But it seems naive to think that size doesn't matter **, so it's slightly disturbing that when I think in terms of incentives, I'm ending up wondering why I should care about my class size at all.
* I do see a potential direct benefit for having my large graduate project class; some student projects can get turned into papers, and often students have me take part in turning their project into a paper, so I may get some research benefit from having a large graduate class. It's not clear that's a big benefit, but at least it's demonstrable.
** Yes, we all knew that was coming before the end of the post....