Ffmpeg

960x540 horizontal 540x960 vertical

ffmpeg -i <input>.mp4 <output>.webm

ffmpeg -i MVI_0083.MP4 -s 960x540 20190322_out.webm
ffmpeg -i MVI_0083.MP4 -s 960x540 20190322_out.mp4
ffmpeg.exe -i "test.avi" -c:v libx264 -crf 20 -c:a aac -strict -2 "test.mp4"

You might want to use the -ss and -t options to create short test encodes. -ss being the start time and -t being the duration — both in seconds. You probably don’t need to encode the entire video to determine what’s acceptable.