The App Store app icon size
App Store Connect needs a 1024 x 1024 pixel app icon as a PNG without transparency. Apple applies the rounded corner mask itself, so the file should be a full square.
How to make a good App Store app icon
Fill the whole square, with no rounded corners and no transparent areas. Keep the design simple enough to recognize at the small sizes the icon is shown at on a home screen.
Questions
What size is a App Store app icon?
App Store Connect needs a 1024 x 1024 pixel app icon as a PNG without transparency.
Should my App Store icon have rounded corners?
No. Upload a full square; the system applies the corner mask.
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.