Here's what I did (feel free to correct me):

1. I have a movie in DivX 3 Low-Motion and AC3 sound
2. I separated the movie into an AVI file (same codec) and a WAV, because my DivX player doesn't play AC3 sound.
3. I downloaded subtitles in .sub format and converted it to .ssa format with SUB2SSA.
4. I made an AVISynth file that A) imported the AVI file, B) added a black border to the bottom of the video, and C) added my .ssa subtitles by importing the VirtualDub Subtitler filter. I made sure the subtitles were on the black bar so they don't interrupt the picture.
5. I opened up the AVS file in Virtualdub. Under the Video menu, I chose "Direct Stream Copy". Under the Audio menu, I opened my WAV file (assuming that VirtualDub will attach the file to my video) and chose Direct Stream Copy.
6. I saved as AVI.

Here's where my problem comes in. Even with both audio and video on Direct Stream Copy, the estimated file size came out to be over 92 GIGABYTES!! That's ridiculous, especially since my source video and audio files were 514mB and 183mB, respectively. Does anyone know how I can get the file down to a reasonable size?