iPhone 14 Plus screen size and resolution
The iPhone 14 Plus has a 6.7-inch display at 1284 × 2778 pixels, an aspect ratio of about 19.5:9. 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
1284 × 2778 is one of the sizes App Store Connect accepts for large iPhones, so screenshots from the iPhone 14 Plus can be uploaded as they are. If you place them in a mockup, set the frame to 1284 × 2778 as well and the result stays within Apple's sizes.
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.
