I had some trouble getting a full size 12.9″ iPad Pro screenshot with the Xcode 9 Simulator. I was using a 1920 x 1200 resolution HP monitor, so I was running the simulator window with 50% of scale. iTunes Connect checks the resolutions when submitting an app and the screenshots should be 2048 × 2732 pixels. My screenshots had wrong resolutions and they were rejected.
I deselected the “Optimise Rendering for Windows Scale” option under the Simulators debug menu. When I pressed CMD + S to take a screenshot I finally found a full 2048 x 2732 resolution PNG image from my Desktop. Problem solved!