Groups mobile device hits to your site by the screen size of the device. The report shows each screen size, the number of visitors to your site that used each screen size, and each screen size as a percentage of the total views. The Screen Size report shows both the height and width of the screen size as compared to the Screen Height report and Screen Width report.
The reported screen size does not indicate the orientation of the device. Regardless of screen orientation, each device has a fixed screen resolution in the report. This size is based on research that determines which orientation is more likely. This means you might see sizes such as 768x1024 and 1024x768 in the same report with each size representing one or more different devices.