How to get lower resolution image in current clipboard after using macOS screen-capture shortcut?
With the image at Clipboard, you can open Preview.app, hit command+N.
Go to Tools menu > Adjust Size..., make modifications.
Using the selection, cover the whole image, liberally. Don't worry if you've got lot of padding around it.


Hit command+C and paste wherever you want.




"Export as..." also provides some options to downsize the image.