@HLTc... you've been a member long enough to know ken_b is not a troll... and ken_b, lighten up!
I've been at this (including ASCII based BBS) since 1985 and remember the angst we had between 640x480 and 800x600. That said, when screen res goes up, and with the "graphic window" based OS has followed, there are some who open as many windows as possible, and some who routinely view full screen.
What needs to be considered, more than anything, is WHO IS VIEWING? How old are their eyes? Are they forced to browser zoom just to read? Have they decided that's too much trouble and have set a personal minimum (an color, etc.) in the browser which overrides the webmaster's pride and joy?
I run 1920 by 1080. Lots of real estate, but my eyes aren't what they used to be so Zoom Zoom Zoom 90% of every site these days. Browser is either 800ish or full screen, depending on the content to be viewed, or whether those other windows I have open are more important.
A fixed site coded to 1200+ and I'd miss quite a bit... most of which will probably be adsense. (Don't see that anyway, personally browse with NoScript active)
One can go back to "the day" with a "Best Viewed Full Screen" reminder prominently displayed, or one can code with the full knowledge that some percentage---I'm guessing about 40%---will not see the site in full glory. And if they have to zoom it up to be legible, then site is "busted" anyway. All these caveats aside, there's nothing wrong with coding in anticipation that at least 60%---again a guess as to how many full screen---will see the site as intended.