Is it possible to resize video in iMovie? I took a screen recording from my iPhone 5 and I want to convert its size from 640x1136 to 750x1134. For extra pixels I need black fill color. Is it possible in iMovie in OS X?
It's not possible to produce that exact resolution in iMovie 10, because it doesn't support custom resolutions. The only resolutions you can export a video project to are:


480p: 854x480
540p: 960x540
720p: 1280x720
1080p: 1920x1080


Note that all of these are 16:9 (1.778) whereas the target you're after is about 3:2 (1.66).

You can certainly add black borders or otherwise crop/frame videos in iMovie, so it could be a stepping stone to another tool. But It's probably not the tool you want if that precise output format is required.

Also remember that many video players will automatically resize and add black borders on playback to best fit the screen's aspect ratio.