This is what you do (this will work if you have at least 1 eligible device):
- Record, edit and finish the App Preview with the device you have.
- Export as a file.
- Go to your Simulators and print screen 1 shot on each the different sizes of iPhone.
- Create new App Preview in iMovie.
- Insert the screenshot of the desired size FIRST, then add the file of the App Preview you’ve already made.
- Export using Share -> App Preview
- Repeat step 4 to 6 for new sizes.
You should be able to get your App Preview in the desired resolution.