iPhone XS Max screen size and resolution
The iPhone XS Max has a 6.5-inch display at 1242 × 2688 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
1242 × 2688 is one of the sizes App Store Connect accepts for large iPhones, so screenshots from the iPhone XS Max can be uploaded as they are. If you place them in a mockup, set the frame to 1242 × 2688 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.
