Resize an MP4 with AAC/H.264 encoding in 16:9 to a smaller 16:9 file
Use the scale filter:
ffmpeg -i input.mp4 -vf scale=-2:720 -c:a copy -movflags +faststart output.mp4
-2 means to automatically choose a value that preserves aspect, and makes the value divisible by 2 (needed for libx264).