Get resolution of a Mac OS X application window I'm using a Macbook Pro Early 2015 running Mac OS X El Capitan.

I'd like to measure the resolution (or at least the aspect ratio) of a specific application window on Mac OS X. Is there a way to do this? The reason I need to do this, is to ensure that I am running the application window at a specific resolution, or aspect ratio.
You can do that with Accessibility Inspector, Xscope or via applescript "get the bounds of the front window"