I just captured a video of my new app running on an iPhone 6 using QuickTime Player and a Lightning cable. Afterwards I created an App Preview project in iMovie, exported it and could successfully upload it to iTunes Connect.
Apple requires developers to upload App Previews in different resolutions dependent on screen size, namely:
ffmpeg -i input.mp4 -acodec copy -vf scale=1080:1920 output.mp4
ffmpeg -i input.mp4 -acodec copy -crf 12 -vf scale=1080:1920,setsar=1:1 output.mp4 did the trick!
Turns out that ffmpeg tries to be smart about aspect ratio, so that the actual size is 1079x1920 with an aspect ratio of 2000:2001.
setsar=1:1forces an aspect ratio of 1:1 and hence, the right resolution
-crf 12as desired quality roughly results in the same file size as the original file created by iMovie, so it should be a safe bet bit rate-wise