Apple said we can use iMovie to make App Previews, but I can't export the correct size using iMovie I'm trying to create an App Preview for an iPhone app.

I recorded a screen capture of the app in the native resolution of the phone.

Then I imported the video in iMovie and used iMovie to add transitions and text, and to arrange the clips.

According to Apple, App Previews should be 640x1136 for the iPhone 5 portrait size, but the export menu doesn't support this size or any custom sizes.

Apple said we could use iMovie, but this doesn't seem to work. How can I change the export size?
Make sure you go to File->New App Preview (available on iMovie 10.0.6 on Yosemite).