iTunes Connect currently asks for images in the following categories:
iPhone 5.8" Display (optional)
This is 1125 x 2436
pixels. You can create this size image using the iPhone X simulator. Make sure that Debug > Optimize Rendering for Window Scale is unselected. Then go to File > New Screen Shot or press Command+S to save the screenshot to the desktop.
As noted above, this is one is optional.
iPhone 5.5" Display
This is 1242 x 2208
pixels. You can create this size image using the iPhone 8 Plus simulator at 100% scale.
iPad 12.9" Display
That is 2048 x 2732
pixels. You can create this size image using the iPad Pro simulator at 100% scale.