how do I select the width and height of video for a specified bitrate?

[codec]

For example, I got a video sequence in D1 of 4Mbps, now I need to get a bitstream for 500k bps, and hope to got a better
quality.Is there any criteria for me to decide the size of the video?

thanks
jogging

[buzzqw]

these value is ofte computed by doing a "compression test" (:search: )

an encoding is launched not on all movie but on some parts, computing the % analyzed and the output file size you can do some math..

BHH