How can I reduce the size of a webm to 2MB?
Use Avanti with ffmpeg. The former has a job control manager where you drop in multiple files at once and go. You can find webm encoding guide for ffmpeg here: http://trac.ffmpeg.org/wiki/Encode/VP8

Use crf mode encoding and experiment till you get desired size. Set b:v high like 10M and tweak CRF value and if you have no audio, use -an in place of -c:a libvorbis.

Your Avanti GUI should look like this: 



Change the crf number (30 in that pic) till you get the size you want.