@storm ffmpeg -i inputfile.mpeg -s wvga -b:v 600k -c:v libvpx output.webm

"-s wvga" means "resize to widescreen VGA", a moderately sized video but enough details for anything that's not art.
"-b:v" is bitrate for video and while 600k is a little too low in some cases, it's much better than the default around 200k (i.e. kilobits per second). Increase this if someone reports it to be too blocky.

"-c:v libvpx" is just because recent ffmpeg versions default to VP9 instead of VP8 for video codec, and VP9 is extremely slow for encoding in comparison. That'll probably change with better algorithms and hardware support in the future.