As for the monitors themselves, it's much much less about the size and more about the pixels. My primary monitor is a 1920x1200 and I love it. I have three other 'smaller' (in terms of pixels) and I honestly think the fourth one is more of a distraction than a help. I do frequently use the other three all at the same time (code on one, database tools on another, and browser for running/debugging the web pages I'm working on on the third).

My iMac display is an insane 2560x1440 (or something close to it) and I still am badly in need of a second monitor. But I feel that with that many pixels, just one additional monitor would be optimal. Particularly if it were that same size (not that I'm going to shell out the $1000 for that any time soon when the 1920x1080 monitors are so cheap now).