http://gentoo-wiki.com/HOWTO_Create_a_DVD:Encode#tovid

mencoder -of mpeg -mpegopts format=dvd:vaspect=4/3:vframerate=25 -srate 48000 -ofps 25 \
-ovc lavc -oac lavc -lavcopts vcodec=mpeg2video:\
   vrc_buf_size=1835:\
      keyint=15:\
	     vrc_maxrate=9800:\
		    vbitrate=4900:\
			   aspect=4/3:\
			      acodec=ac3:abitrate=192 \
				  ${VIDEO}.avi -o ${VIDEO}.mpeg2


mencoder -of mpeg -mpegopts format=dvd:vaspect=16/9:vframerate=25 -srate 48000 -ofps 25 \
-ovc lavc -oac lavc -lavcopts vcodec=mpeg2video:\
   vrc_buf_size=1835:\
   keyint=15:\
   vrc_maxrate=9800:\
   vbitrate=4900:\
   aspect=16/9:\
   acodec=ac3:abitrate=192 \
${VIDEO}.avi -o ${VIDEO}.mpeg2


Unlike AVI files, etc PAL and NTSC can only display specific aspect ratios. Full PAL and NTSC resolutions are 720×576 and 720×480, respectively. For example, if one has an AVI file with a resolution of 624x268 (2.35:1). To get an DVD-Video-ready MPEG-2 without any squashing or squeezing of the video you would add black bar padding to the top and bottom of the MPEG, shown below
ffmpeg -i input_file.avi -target pal-dvd -s 720x440 -padtop 68 -padbottom 68 -aspect 16:9 movie.mpg