With Handbrake, I could only get 1333x750. I tried to follow the advice given on ffmpeg resizing video puts it at wrong resolution to force the aspect ratio to 1.79.
Apple however, is not accepting the file - some issues with the bitrate and stuff. Any other advice?
Check this answer from @Benjamin Thiel
I added just for documentation.
ffmpeg -i input.mp4 -acodec copy -crf 12 -vf scale=1080:1920,setsar=1:1 output.mp4 did the trick!
Turns out that ffmpeg tries to be smart about aspect ratio, so that the actual size is 1079x1920 with an aspect ratio of 2000:2001.
setsar=1:1 forces an aspect ratio of 1:1 and hence, the right resolution -crf 12 as desired quality roughly results in the same file size as the original file created by iMovie, so it should be a safe bet bit rate-wise