(fileseizeMB * 1024 *

Lets say you had a 1h30m movie, and you wanted to fit it into 128MB.
(128 * 1024 *

If you are encoding with AVI (DivX), you need to set aside some bits for the audio. Lets say you had 32Kbps audio. (Not needed for MPEG1)
194 - 32 = 162Kbps
This general method works for any application you can stick a bitrate into (Practically anything).
So if you were encoding MPEG1 the final bitrate would be 194Kbps, if you were encoding with AVI, the final video bitrate would be 194Kbps minus the audio.
Note for AVI encoding: Always use 2-pass, for the allocated bits make a better looking movie.
If all you wanted to know is how to encode a movie into 128MB then theres your answer, if you want to know how to actually encode a movie then you should find a tutorial on the program you intend to use.