The App Store screenshot size
App Store screenshots must match one of the exact sizes App Store Connect accepts for a display size. For iPhone, the 6.9 inch sizes (1320 x 2868 or 1290 x 2796) or the 6.5 inch sizes (1284 x 2778 or 1242 x 2688); for iPad, the 13 inch sizes (2064 x 2752 or 2048 x 2732). Landscape is the same sizes turned sideways. Apple scales them down for smaller devices.
How to make a good App Store screenshot
The first two or three screenshots show in search results, so make those count: one benefit each, in large text, with the app screen underneath. Use PNG or JPEG without transparency.
Questions
What size is a App Store screenshot?
App Store screenshots must match one of the exact sizes App Store Connect accepts for a display size.
Which App Store screenshot sizes are required?
One set for the largest iPhone display (6.9 inch, or 6.5 inch) and, if your app runs on iPad, one for the 13 inch iPad. Apple scales them for smaller devices.
Can I upload landscape screenshots?
Yes, the same sizes turned sideways are accepted.
Is my image uploaded anywhere?
No. The image is read and resized in your browser; nothing is sent to a server.
How does the resize work?
It scales the image to cover the target size and crops the overflow evenly from the edges, so nothing is stretched.