Honestly I think more, smaller, monitors are better than fewer big ones. Given my choice I'd happily take 3 22" monitors or even 2 21" monitors + 1 19" monitor if push came to shove. I can usually see all the code I need in one 21" monitor just fine, but usually need another one for reference of some kind: docs or watching debug output, etc. The 3rd would be for collaboration: chat windows & email. If you can isolate distractions to their own monitor, they're easier to ignore while busy and require less window management and never cover up your real work.

Also in general it's more helpful to talk about monitors in terms of resolution rather than size; it's entirely possible to end up with a 27" monitor that is the same resolution as a 24" monitor, which means it doesn't actually have any more usable space, everything is just bigger, and that isn't very useful unless your devs are visually impaired.