iPhone XS screen size and resolution
The iPhone XS has a 5.8-inch display at 1125 × 2436 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
1125 × 2436 is one of the sizes App Store Connect accepts for medium iPhones, so screenshots from the iPhone XS can be uploaded as they are. If you place them in a mockup, set the frame to 1125 × 2436 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.
