Browser is always sized to 1024x768
Most sites are still designed to either 800x600 or 1024x768. If I leave the browser maximized, the lines of text on many of these end up too long to read comfortably. So I guess technically the browser size has nothing to do with the resolution the monitor is set at, but usually when someone asks this question they're interested in how much real estate they can design to.
This is my sig. There are many like it, but this one is mine.