Check out How To Resize Photos With a Shortcut On Your iPhone or iPad at YouTube for closed captioning and more options.
This is the simple one-file shortcut:

This is the shortcut that works with multiple files:

Download it here:
Resize Photo Shortcut
Thanks for this shortcut. I'm wondering if I'm missing something on the Mac side. I have the shortcut, which works fine under ios, showing up on my mac as well. However, when I click on an image in the finder, or in Photos, I don't have the Save to Files option for saving. Is that a feature of IOS only?
Dan: macOS is very different. This Shortcut is for iOS. macOS doesn't have a simple "Files" app as handling files is much more central to using a Mac. You'd need a different Shortcut. But of course you have the Preview app on the Mac so you can just use that to resize an image. You don't have that on the iPhone thus the need for a Shortcut like this.
Got it. Thanks. Always looking for that universal shortcut. :-)