>>29260693
Just use AVIDemux. It now supports webm, vp9 (libvpx) and vorbis, with options to specify video size before it renders. It generally achieves a pretty good result.

Issues start occurring due to sound file size more often than video, in my experience, at which point you want to use audacity to crush the bitrate, and even then... Also, videos that are more "busy" will require higher bitrates to not look like shit.