Just for your info here are the commands I have used for ffmpeg and avconv on my machine. I'm not sure about sound on these, but I'm sure you could build on those commands for sound if you need it, or you could change the output resolution which I reduced to half my screen size just to show you how it can be done .
will make an avi with resolution of 720x450, ie half my screen resolution of 1440x900, and at the default refresh rate (25fps?)
avconv -f x11grab -s 1440x900 -i :0.0 -s 720x450 -q 5 Desktop/video.avi
the -r 15 reduces refresh rate to 15fps, but otherwise all is the same.
avconv -f x11grab -s 1440x900 -r 15 -i :0.0 -s 720x450 -r 15 -q 5 Desktop/video.avi
note the minor change to the command output quality of -sameq which does exactly what it suggests, makes output the same quality as input.
ffmpeg -f x11grab -s 1440x900 -r 15 -i :0.0 -s 720x450 -r 15 -sameq Desktop/video.avi