$ DISPLAY=:1 xdpyinfo | grep dimensions
  dimensions:    1920x1080 pixels (488x274 millimeters)


$ /usr/bin/ffmpeg -y -f x11grab -i :1 -framerate 25 -video_size cif -f alsa -i hw:Loopback,1 -c:v libx264 -pix_fmt yuv420p -b:v 2500k -c:a aac -strict -2 -b:a 192k -movflags +faststart -preset ultrafast -report /opt/myApp/output/DSkMoMue8HTbNkVtvy2id6.mp4

....
Opening an input file: :1.
[x11grab @ 0x6cfea0] device: :1 -> display: :1 x: 0 y: 0 width: 640 height: 480
[x11grab @ 0x6cfea0] shared memory extension found
[x11grab @ 0x6cfea0] All info found
[x11grab @ 0x6cfea0] Estimating duration from bitrate, this may be inaccurate
Input #0, x11grab, from ':1':
  Duration: N/A, start: 1427743112.160941, bitrate: 294617 kb/s
    Stream #0:0, 1, 1/1000000: Video: rawvideo (BGR[0] / 0x524742), bgr0, 640x480, 294617 kb/s, 29.97 tbr, 1000k tbn, 29.97 tbc