Comment 6 for bug 589485

The 'screen size' as reported in the core protocol now respects the DPI value given by the user or config file and ignores the actual monitor DPI of any connected monitor.

The real monitor size is reported through the RandR extension if you really need to know the physical size of the display.

This is intentional and follows the practice seen in many other desktop environments where the logical DPI of the screen is used as an application and font scaling factor.

If you don't like it, you're welcome to configure the X server to taste; we haven't removed any of those knobs. And, you can even use xrandr to change things on the fly.