Apparently, Apple documentation can't keep up with App Store changes. Until today, biggest (optional) display size was 5.8-Inch Super Retina Display with resolution of 1125 x 2436 pixels. Currently it is 6.5-inch display, but 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.
New size is 1242 × 2688 pixels