Creating iOS app preview in iMovie but getting iTunes Connect Error
Here is a trick using just iMovie and Quicktime to get the right dimensions.


Use QuickTime to record a short clip from the size of the device you need
Use iMovie and insert that clip into a new "App Preview" project
Add your actual video after the short clip


Now, any clips that you export will be in the right dimensions. 

If you need to hide your short clip, you can shorten it to 0.1 seconds. Put a title over it and delete the text. Now your video starts with 0.1 seconds of a black screen.