1. Open the movie in VirtualDub.
2. Click on Video > Full Processing Mode
3. Click on Video > Compression...
4. From the list box, select DixX Pro (tm) 5.2.0 Codec (or later version if you have)
5. Click the Configure button.
6. The DivX codec configuration page will open.
7. Click on the Video tab and check Enable Resize
In the General tab you'll find Bitrate Calculator button. Use it to calculate desired bitrate. It will give you approx bitrate for your desired filesize.