For years, I used Stomp for resizing videos under OS X. However, in addition to recently being retired, it has had trouble processing a number of formats of late, either hanging altogether or displaying lengthy time estimates (e.g., ~6 hours to scale a 6 minute video by half).
Happily, ffmpeg makes quick work of scaling video while preserving the aspect ratio, e.g.,
$ ffmpeg -i in.mp4 -vf scale=640:-2 out.mp4
UPDATE: To optimize for web streaming, add -movflags faststart. To optimize existing videos for streaming without have to reencode, use ffmpeg -i in.mp4 -movflags faststart -acodec copy -vcodec copy out.mp4
/mac | Apr 28, 2018