It all depends on your settings, you'll have to play around with them to see how good they look vs size

These are the settings I use for x264

ffmpeg -i infile.mp4 -channel_layout 5.1 -c:v libx264 -vf yadif -crf 28 -force_key_frames "expr:gte(t,n_forced*4)" -x264-params log-level=none  -c:a aac -b:a 192k -preset -preset fast -strict experimental -loglevel warning -stats  outfile.mp4