How do I export my video in decent quality, but also keep the file size down?
Here's how I do it:
1. File --> Save Movie File
2. Choose "My Computer", click "Next".

3. Enter a name for the movie file and click "Next".
4. Choose this video setting:

In my experience, "Video for local playback (2.1 Mbps)" has been the best setting. Slightly smaller .wmv files than the "High quality Video (large)" setting, but also better quality in general. 2D elements like ammo display, FPS counter, etc are kept more intact and are readable in more frames than with "High quality Video (large)" setting.
