The current appstream spec recommends this for screenshot sizes:

Ideally, all image screenshots as well as videos should have a 16:9 aspect ratio, and should have a width that is no smaller than 620 pixels.

This is not what we recommend to apps anymore, for a variety of reasons:

• Forcing apps into 16:9 often looks weird
• App store frontends have gotten better at dealing with screenshots with different sizes
• It's a pain for app developers to resize their app to this specific size
• It encourages large window sizes, which is the opposite of what you want here
• 620px width doesn't make sense for all apps, e.g. small utilities. Also, mobile screenshots are smaller than that by necessity.

I'd change this guidance to something more like this:

• Use an aspect ratio that works for the app's UI
• Generally try to keep the window size small to make the content more visible when it's scaled down in app store frontends
• Use window screenshots with baked-in shadows
• Use the default GNOME stylesheet, icon set, font, and window decorations

cc @pwithnall