There are calculators out there. The same bitrate can yield way different qualities, depending on resolution. So you have to determine the resolution you are looking to output, then figure out what level of compression is adequate to produce the quality you are looking for and that will give you a filesize.
Conversely, if you're looking for a specific filesize, you're going to have to sacrifice either resolution or compression.
It's sort of a delicate dance.
Gordian Knot can help you with all that, if you have the file in a .avi format, or have a .d2v or .dvs for it.
You can also calculate it with the xvid "configuration" deal that you get when you set the settings for the xvid codec.
You can also figure it out with one of the many bitrate calculator tools out there, such as
Bitrate Calculator | DivX Labs - Everywhere Communication Occurs Community Happens .
My suggested workflow would be:
1. Capture and edit in videostudio.
2. Output final edited video in huffyuv or something of that nature (basically anything losslessly compressed).
3. Determine desired bitrate and resolution required for the video.
4. Open it up in virtualdubmod.
5. Add a resize filter (Video..Filter..Add...Resize...OK) to select the size of the final video
6. Save as...XVID/DIVX etc, set your bitrate or filesize
7. Open a beer or 10 and drink while you wait
Done.
This saves you from having to go through, what appears to be 2 MPEG-2 processing steps, which will cost you quality each time. It should also save you a considerable amount of time.