Find size of a window When I have a window for an application open I would like its size, height and width. Is it 900 * 625, 716 * 456 or what?

The reason I need to know is just for some usability testing. 

Any tips how I get this info?
If you want it in a UI use Accessibility Inspector. It's in Xcode &gt; Open Developer Tool &gt; Accessibility Inspector (It requires to install Xcode).