Google Pixel 3 screen size and resolution
The Google Pixel 3 has a 5.5-inch display at 1080 × 2160 pixels, an aspect ratio of about 18:9. A full-resolution screenshot from it comes out at exactly that size, and that's the image you drop on the mockup's screen.
Using it on Google Play
It passes Google Play's 2:1 rule, but to appear in recommendations Google wants 9:16 images of at least 1080 × 1920. A mockup frame at that size covers it.
