The following formula should not be treated as a hard and fast rule but as general guidance for newbies.
kbps to use = (video width x video height x framerate x constant) kbps / 1024
A constant of 0.25 will usually give you a good balance between quality and compression, so for instance, if you've got a video of 320 x 240 with a framerate of 29.97, applying the formula, we get:
kbps to use = (320 x 240 x 29.97 x 0.25) kbps / 1024
= 562 kbps
I usually get away with 500kbps or less by setting Partition Decision to "7B Ultra High (RDO on B-Frames)" and Method to "Uneven multi hexagon" in Avidemux.
Good luck and have fun. :-)