That said, measuring pixels is also probably not really looking at what's really important here.

Pixels seems to almost definitely be the right measure, here- basically, you want to have N windows open with enough information in all of them. For most tasks, N is greater than 2, but once you have enough windows extra space is just wasted. (One of my former labmates had four monitors, which generally had Matlab open in one window, the docs in another, his system monitor in a third, and nothing in the fourth- so he probably would have been fine with two monitors most of the time- but he did sometimes do something involved enough that he needed all four. For someone working on code where lots of different things all matter, having lots of monitors seems useful.)