ffmpeg -i output.avi -acodec libvorbis -ac 2 -ab 96k -ar 44100 -b 345k -s 1366x768 -crf 20 -minrate 700k  output.webm

user@user-K56CM:/tmp$ ffmpeg -i output.avi -acodec libvorbis -ac 2 -ab 96k -ar 44100 -b 345k -s 1366x768 -crf 20 -minrate 2000k  output.webm
ffmpeg version 0.8.9-6:0.8.9-0ubuntu0.13.04.1, Copyright (c) 2000-2013 the Libav developers
  built on Nov  9 2013 19:09:48 with gcc 4.7.3
*** THIS PROGRAM IS DEPRECATED ***
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
Input #0, avi, from 'output.avi':
  Metadata:
    encoder         : Lavf53.21.1
  Duration: 00:00:05.13, start: 0.000000, bitrate: 6072 kb/s
    Stream #0.0: Video: theora, yuv420p, 1366x768 [PAR 1:1 DAR 683:384], 30 tbr, 30 tbn, 30 tbc
    Stream #0.1: Audio: pcm_s16le, 48000 Hz, 2 channels, s16, 1536 kb/s
File 'output.webm' already exists. Overwrite ? [y/N] y
[buffer @ 0x1c82940] w:1366 h:768 pixfmt:yuv420p
[libvpx @ 0x1c80160] v1.1.0
Output #0, webm, to 'output.webm':
  Metadata:
    encoder         : Lavf53.21.1
    Stream #0.0: Video: libvpx, yuv420p, 1366x768 [PAR 1:1 DAR 683:384], q=-1--1, 345 kb/s, 1k tbn, 30 tbc
    Stream #0.1: Audio: libvorbis, 44100 Hz, 2 channels, s16, 345 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
Press ctrl-c to stop encoding
frame=   44 fps=  2 q=0.0 Lsize=    1446kB time=5.13 bitrate=2307.9kbits/s
video:186kB audio:1253kB global headers:4kB muxing overhead 0.226153%