I can’t tell if this is an easy or hard issue. I’ve never had to do it. I typically am using the default CMD-Shift-3 command.
I want to share some screenshots on Twitter and Twitter recommends the images be 1024x512 (2x1 ratio).
Is there a way on a Mac to take a screenshot with that specific pixel dimension/ratio? I’d like to have the bounding box set so that if I have to, I can resize the browser window to make the content fit within the screenshot.
Check Keyboard Maestro maybe? 
You can hold down Command + 4 then click + drag to create a region on your screen that matches the dimensions you referenced.
I use Command + 4 to get a rough measurement of things on my screen all the time. Just hit the escape key before you click or release the keys to avoid actually taking the screenshot.
You can move the region once you choose your screenshot size as well.
Here’s a good source for more details: Take better screenshots on macOS.
I hope this helps!
-dm