Apple - Apple said we can use iMovie to make App Previews, but I can't export the correct size using iMovie

Make sure you go to File->New App Preview (available on iMovie 10.0.6 on Yosemite).

Add an image to the beginning of the video, e.g. one second splash screen or 0.1 second black/white screen. Make sure the image dimensions match the intended device dimensions. Video added after the image will conform to these dimensions - you will have a resizer option in the preview video to change your video size to match the initial screen dimensions.

You can also remove the beginning image after all of this and your movie will retain the new size.

If like me and the OP you did all of the work to create your preview only to find that you can't 'share' it as a preview because you did not start it as a preview then here is the answer.

Start a 'New App Preview' then open your original project -> click in the timeline and select all (cmd + a) -> copy that (cmd + C) -> go to your empty app preview -> click in the timeline and -> paste (cmd + V).

All your hard work is now in the app preview video.