Open your source file in FitCD, then set FitCD's resolution setting to 'VCD' and the resize mode to 'precise bicubic'. Enable KVCD prediction (tick 'file size prediction'). To the left there's a text-field with the .avs-script in it. If you've got an older version of FitCD, change "Framecount" to the movie's frame-count

and "Framerate" to your movie's frame-rate (if it's PAL write 25, if it's NTSC write 29.97, or if it's 3:2 pulldown write 23.976). Now save this script and open it with TMPGEnc. Load the LBR template and do your first encode. In the meantime you can open KVCD Predictor, set the length of your movie (given in FitCD) and set your preferred audio-bitrate (I'd take 128 or 160). Now you're given the filesize which your sample encode should have. Reencode your sample with optimized settings (increase or decrease the quality setting) until the size of your sample encode more or less matches the size that KVCD predictor has calculated.
Best regards