ffmpeg -f x11grab -s 1824x984 -i :0.0 -s 800x600 -an -f flv "rtmp://live-arn.twitch.tv/app/live_***"
ffmpeg version N-89616-ge6a1dfc9ce Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 6.3.0 (Raspbian 6.3.0-18+rpi1) 20170516
  configuration:
  libavutil      56.  7.100 / 56.  7.100
  libavcodec     58.  9.100 / 58.  9.100
  libavformat    58.  3.100 / 58.  3.100
  libavdevice    58.  0.100 / 58.  0.100
  libavfilter     7.  7.100 /  7.  7.100
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
[x11grab @ 0x29a7240] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0.0':
  Duration: N/A, start: 1514382396.166636, bitrate: N/A
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1824x984, 29.97 fps, 1000k tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> flv1 (flv))
Press [q] to stop, [?] for help
Output #0, flv, to 'rtmp://live-arn.twitch.tv/app/live_***':
  Metadata:
    encoder         : Lavf58.3.100
    Stream #0:0: Video: flv1 (flv) ([2][0][0][0] / 0x0002), yuv420p, 800x600, q=2-31, 200 kb/s, 29.97 fps, 1k tbn, 29.97 tbc
    Metadata:
      encoder         : Lavc58.9.100 flv
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
[flv @ 0x29af6a0] Failed to update header with correct duration.te= 208.7kbits/s speed=1.21x
[flv @ 0x29af6a0] Failed to update header with correct filesize.
frame=   22 fps=2.4 q=2.0 Lsize=     263kB time=00:00:10.31 bitrate= 208.7kbits/s speed=1.15x
video:262kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.209773%
Exiting normally, received signal 2.