Apparently, Apple documentation can't keep up with App Store changes. Until today, the biggest (optional) display size was a 5.8-Inch Super Retina Display with a resolution of 1125 x 2436 pixels. Currently, it is a 6.5-inch display, but the resolution is nowhere to be found (docs at https://help.apple.com/app-store-connect/#/devd274dd925 only mention 5.8-inch display). App store connect doesn't mention the resolution and error message "The dimensions of one or more screenshots are wrong. Learn More." redirects to docs linked above.