ffprobe

If you just want to get the size of the video then you can get that, and other info, directly with ffprobe. This will avoid redirection, temporary output files, and the additional parsing.

$ ffprobe -v error -select_streams v:0 -show_entries stream=height,width -of csv=p=0:s=x input.mkv 
  1280x720

See FFmpeg Wiki: FFprobe Tips for more examples.

tee

For users who want to encode and capture the resulting console output, I recommend using tee. The problem with pure redirection is that important messages such as error messages, failures, and prompts can be missed.

You can avoid this by including tee to show the output in the console and to save it to a file:

ffmpeg -i input … output |& tee console.txt

ffmpeg outputs to stderr instead of the more typical stdout, so the & is added to the | pipe to deal with that. This is only for Bash 4+. If you're using something else then change |& to 2>&1 which redirects stderr to stdout before it is sent to the pipe.

Somewhat better idea is to use ffprobe

ffprobe -show_format -print_format json TheNorth.mp4

that will output JSON formated info about video. Guess it is easier to parse than raw output. To redirect output to file use just ordinary pipe > result.txt similar to accepted answer but without two.