Meazure - What are the dimensions of that image? How big is this window?
ShareX - ShareX is a free and open source program that lets you capture or record any area of your screen...
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.
LightShot - The fastest way to take a customizable screenshot.