I'm making a sticker pack for iOS, and before I can submit it to the App Store, I need to include at least one screenshot of the app from a 5.5" iPhone, and a 12.9" iPad Pro. These are both devices that I don't have.
I'm also not that well versed in photoshop, but I could learn it as I have read that that's a way to make screenshots to use for the App Store.
Thank You for your help!
You could use simulator. With your app opened in simulator press cmd+S or from menu: File -> Save Screen Shot. Screen shot will appear on your desktop.
UPD: as opposed to https://stackoverflow.com/a/41730054/2305175 you'll get ready to use screen shot with size exactly as in simulator without further work like cropping it
Note: Screenshots resolution is independend of your monitor resolution. So this even works when your monitor has lower resolution than what you need for the screenshot.