I'd play with ffmpeg:
ffmpeg.exe -i "original_file" -ar 22050 -ab 32 -cmp rd -subcmp rd -mbcmp rd -ildctcmp rd -precmp rd -mbd rd -skipcmp rd -qscale 15 -r 26 -f flv "output_file.flv"
Basically all flv encoders use ffmpeg. The difference is you will have more options when using it directly (for example the *cmp use more computing time but give better output).
This command-line is a bit old I think. I have a newer one but stored on another computer I can't acces right now. I'll post it tomorrow.
Moreover web-based encoders have an important load and therefore need to use "fast" settings generally making the picture worse than normal.
I think you can count on a 1/6 ratio depending of the quality of your source (for example, videos recorded with digital cameras at ski are usually very bad and badly compressed because of all the artifacts). (this is my personnal experience for some videos with my classmates)
Is your source file already compressed ?
This post has been edited by Camarade_Tux: 10 November 2007 - 06:10 PM