1320×700
ffmpeg -f x11grab -show_region 1 -r 10 -s 1320x700 -i :0.0+15,25 -vcodec libx264 -qscale 0 -y out.mkv
1250×780 + VGA
ffmpeg -f x11grab -show_region 1 -r 10 -s 1250x780 -i :0.0+1380,50 -vcodec libx264 -pix_fmt yuv420p -q:v 0 -y out.mkv
or
ffmpeg -f x11grab -show_region 1 -r 16 -s 1320x700 -i :0.0+15,25 -vcodec libx264 -pix_fmt yuv420p -preset medium -y itk.mkv