As I said, it's the 27". Too big for one window, too small for 2. Its pixel density has the same problem by the way: it's denser than the usual low-density HD screens, but it's also nowhere near "retina". I have a low-cost HP Pavillon 23" HD monitor on both sides of it.
I've now got my 2011 22" iMac sitting next to two 22" Dell UltraSharps, and it's amazing for my workflow :)
That said, I also do a heap of work on my 11" (1366x768) HP notebook, so I'm crazy, and use so very many virtual desktops its not even funny.