I think the region of the screen I can concentrate on at once is quite small, maybe twelve words wide and six lines tall. But then why do I prefer two large high-resolution screens to one low-res screen? I can't really use all that screen space, can I?
Let's talk about the navigation features provided by human vision. Although I have this 12x6 mental "window", the window doesn't stay still. I can read a whole screenful of text without touching the mouse. Human vision has built-in scrolling. I can take in more of the screen at once by sacrificing attention to detail; or I can concentrate on one line of code, at the cost of ignoring everything else for a moment: visual zoom. If I look away, then when I look back, my eyes easily snap to where I was. This is kind of like Alt+Tab in that it lets me hop back and forth between different things; call it visual switching. If I need to look for something, I'll zoom out, skim down without reading, find it, and zoom back in, all without thinking: visual search.
With a huge screen, I can scroll, zoom, switch, and search without touching the keyboard or mouse. With a smaller screen, I have to use the mouse or keyboard to navigate. Each motion takes time and a little bit of brainpower; and each time the screen changes, it takes me a moment to find my place. Eyeball scrolling is superior to software scrolling. Eyeball switching is superior to Alt+Tab.
Having dispensed with that paradox, I want to talk about the 12x6 window and what it means for PL and IDE design. More to come.