Looking for a screen measuring app for macOS
Cmd-shift-4 brings up a screen capture utility. As you move the new cursor it displays the number of pixels you move in x and y directions. So you can easily measure pixel distance between 2 points. You don't have to capture the screen area, just opt out of that option.

I've also used the app PixelStick ($7 but free trial) that brings up a screen measuring tool. It lists current pixel, the distance between pixels, and the angle between two pixels. You can calibrate the pixel distance to other scales and has a built in map mode. It also has a screen capture button and a color grabber.