Is there an easy way to resize screenshot in macOS markup editor?

Just a quick addition to @NimeshNeema's answer, you can directly open your screenshot in Preview using the button on the top right of the "markup editor"

I don't know about the markup editor but you could open the screenshot in Preview and resize it there. Once in Preview go to the tools menu item and click on adjust size. I'm currently running High Sierra so I don't have access to the markup editor but if you can't do it in markup editor this method will work.

The markup editor doesn't have a control to resize screenshot. Instead you can skip clicking on the screenshot and let it get saved (which is by default on Desktop). Once it is saved, you can open it in Preview.app and use the Adjust Size... command under the Tools menu to resize it.