ffmpeg -i "input.mkv" -c copy -f null /dev/null 2>&1 | tail -n 2 | grep -Po "(?<=video\:)([0-9]+)"
 
or
 
ffmpeg -i "input.mkv" -c copy -f null /dev/null 2>&1 | tail -n 2 | grep -Po "video\:([0-9]+)(?=kB)" | cut -f 2 -d ":

ffprobe -v error -select_streams v:$i -show_entries stream=size -of default=noprint_wrappers=1:nokey=1 "input.mkv" | uniq

ffprobe -v error -select_streams v:0 -show_entries format=size -of default=noprint_wrappers=1:nokey=1 "input.mkv" | uniq

This method reads each video packet, individually, and totals up each packet size.

• Sums up the size attribute of -show_entries command.

video_sizes=$(ffprobe -v error -select_streams v:0 -skip_frame nokey -show_entries packet=size -of default=nokey=1:noprint_wrappers=1 "input.mkv")
 
sum_video_size=0
precision=6
count_video_pkt_size=0
 
echo "Calculating video size..."
for video_size in $video_sizes; do
  (( sum_video_size+=video_size ))
 
  (( count_video_size++ ))
done;
 
size=$( echo "scale=6; ${sum_video_size}" | bc)