Chris Coyier has an amazing article that finally puts the screen resolution myth to rest. Despite screen resolutions getting progressively bigger, the available width for your web designs is not. If you have a ginormous 30-inch monitor, you probably don’t keep your web browser full-screen. You probably have it at a more comfortable size and use the extra space to keep other things visible simultaneously. (This goes double for Mac users, who don’t really have the whole “full screen” habit to begin with.)
The post also demonstrates a way to use a bit of jQuery magic to record users’ browser window sizes and dump it into a database, since Google Analytics does not yet provide this metric.
Practically, it’s still best to use the old “keep it under a thousand pixels” rule. It matches up well with the most common browser window ranges, and many of us have lower screen resolutions anyway. I primarily use a 13″ laptop with a 1200-pixel wide LCD. That means I keep Firefox resized to be around one thousand pixels wide, leaving some breathing room while still being able to view most web sites without annoying horizontal scroll bars.
Screen Resolution ≠ Browser Window [CSS-Tricks]