Usage is: ./tiny-video.bash "/path/to/input/file.mkv" "/path/to/output/file.mp4"

tiny-video.bash

#!/usr/bin/env bash

target_size=$(( 6 * 1000 * 1000 * 8 )) # 6MB
length="$(ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "${1}")"
length_round_up=$(( ${length%.*} + 1 ))
total_bitrate=$(( target_size / length_round_up ))
audio_bitrate=$(( total_bitrate / 5 )) # 20% of the total bit rate
video_bitrate=$(( total_bitrate - audio_bitrate ))
ffmpeg-bar -i "${1}" -b:v ${video_bitrate} -maxrate:v ${video_bitrate} -bufsize:v $(( ${target_size} / 20 )) -b:a ${audio_bitrate} -ac 1 -vf scale=-1:144 "${2}"
ffmpeg-bar -i "${2}" -map_metadata -1 -c:v copy -c:a copy -fflags +bitexact -flags:v +bitexact -flags:a +bitexact "${2%.mp4}-2.mp4"
mv "${2%.mp4}-2.mp4" "${2}"