During my time in the marketing department at Age of Learning, one of my regular tasks was resizing creative assets for various channels in a wide range of sizes.
After I designed a standard square ad layout and had it approved by all involved parties, I would modify the ad layout to work in many different sizes. A similar process was involved in creating graphic for the various app store platforms. They required a wide array of dimensions and sizes to ensure image quality across mobile platforms.
Iterate. Resize. Repurpose.
Our standard workflow for ad campaigns was to create a range of standard square ad designs for Facebook, run those first, and then create set of banner ads for Google and other platforms, based on the best performing creative from Facebook. I excel at resizing assets like this; I pay close attention to make sure every ad looks it best, and I get turn them around quickly.
When an ad design performed particularly well, we often would make a few adjustments to it and run it again, to explore how variations in messaging, offer language, or design elements might effect performance. Regular performance meetings allowed us to collaborate on ideas to keep our numbers high and strive to push them even higher.