In the old version of the Xcode Simulator (Xcode 8), you could press Command + 1, and it would show the simulator to be it's maximum dimensions (100%), even if that involved it expanding past the visible screen space. You could then press Command+S and take a screenshot from any simulator, and use that directly for the app store. (It would be the correct resolution). I did this for iPad Pro 12.9" and iPhone 6s Plus, to quickly fill out my screenshot sections.
Now, in Xcode 9, this no longer works. I have tried all options: manually resizing, setting to 100%, zoom on/off, etc. None of the options allow me to expand the simulator past the visible bounds of my screen. It won't let you drag the corner past the bounds. So when you press COMMAND+S to take the screenshot, it winds up to be lower res than what ItunesConnect needs.
So how can we now take screenshots using the simulator only, and have them be usable for the store with the correct resolution?
In Xcode 9.1, go to Simulator Menu > Debug > Optimize Rendering for Window Scale. disable this option, now the screenshots will be in the right size (resolution).