iPhone 8 Plus screen size and resolution
The iPhone 8 Plus has a 5.5-inch display at 1080 × 1920 pixels, an aspect ratio of about 16:9. A full-resolution screenshot from it comes out at exactly that size, and that's the image you drop on the mockup's screen.
Which App Store screenshot size it fits
App Store Connect doesn't list 1080 × 1920 any more. Put the screenshot in a mockup and export the frame at 1242 × 2208, the nearest size Apple accepts.
For a new app, Apple requires the medium Dynamic Island size (such as 1206 × 2622) and a large iPhone set (1320 × 2868 or 1284 × 2778). One mockup scene exported at both sizes covers it.
