How do I measure the number of pixels in a window?

When I want to use Window Capture, the portion I want to use is 320x180 at 1:1scale, but the actual window is larger, but has the right size and position.

I understand i find the top add 180 get the bottom, and find the left and add 320. And I just plug in numbers until I find where I want.

Also I need to know the Window size, so I can retype it into a Google Chrome extension.

Apparently Google has ways to make windows of certain size. But no way to measure preexisting windows.

The window has to be larger because there are tabs and other features that have to be added. And the crop lines are relative to the window size. So I need to reproduce THIS size window of unknown size before I accidentally lose it.
 
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