I've just created some app video previews for both the iPad and iPhone 6 Plus in iMovie and was able to export these videos to the needed sizes (900 x 1200 for iPad and 1080 x 1920 for iPhone 6 Plus). As a bonus, Apple allows us to use the 1080 x 1920 resolution for 4 inch devices.
That only leaves the iPhone 6 which needs to be 750 x 1334. There is no option to export to this size in iMovie as I don't believe it's a standard size.
I am wondering, how are people producing videos for this resolution?
Use quick toggles to change video resolution and frame rate To display quick toggles on iPhone X and earlier, go to Settings > Camera > Record Video, then turn on Video Format Control. On iPhone 14 models, quick toggles are available in Cinematic mode to switch between HD or 4K and 24, 25, or 30 fps.
Launch Settings on iPhone, scroll down and select Camera, then choose Record Video. Here you can see all the available resolutions or size you can choose from.
1) Download HandBrake (open source video transcoder)
2) Open HandBrake, click picture settings in the top right and enter 750 x 1334 for the resolution and close the window.
3) Change the anamorphic setting to none in picture settings (or else it will steal one of your pixels and make it 1333 x 750).
4) Click start in the upper left of the main window and it will save your new video to the desktop.
Edit: Updated to reflect fix mentioned below to prevent losing one pixel on the width (thank you).
As @quemeful mention solution above can give you 1333x750 video. To get strict 1334x750 resolution use anamorphic: none in picture settings.
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