avconv -i infile.mkv -c:a copy -c:v copy outfile.avi

This will not modify the quality.

avconv -i infile.mkv -c:a ac3 -ab 96k -ac 2 -c:v mpeg4 -s 640x352 -b 800k -bufsize 20M outfile.avi

This should change size to ‘ripped’ avi of medium quality (not tested).

https://askubuntu.com/questions/520584/how-can-i-convert-a-9-5-gb-mkv-file-to-mpg-or-avi-with-avconv