getScreenResolution()
return dots-per-inch (DPI). If you want the size of the screen, just use getScreenSize()
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Here's a quote that gets you the basic idea of DPI vs. pixels:
So, if you have a 600 pixel x 600 pixel image at 300DPI, it will output at 2 inches square. If you change this images DPI to 150, this will mean it will output at 4 inches square. So, as you can see, changing the DPI of an image changes it output size.
You can find rest of the explanation here.