To re-encode an average 45 minutes TV-Series episode to about 1.4GB (mkv, x264, 720p), use this:
ffmpeg -y -i <infile> -acodec ac3 -scodec copy -s hd720 \ -vcodec libx264 -preset slow -threads 0 -x264opts \ fast_pskip=0 -b:v 4000K -minrate 4000K \ ./out.mkv