iPhone 12 mini screen size and resolution
The iPhone 12 mini has a 5.4-inch display at 1080 × 2340 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
1080 × 2340 is one of the sizes App Store Connect accepts for medium iPhones, so screenshots from the iPhone 12 mini can be uploaded as they are. If you place them in a mockup, set the frame to 1080 × 2340 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.
