The easiest, no fuss method is to export your timeline into one clip on to your hard drive. Open IMovie and import it into there. Drag it down to the IMovie timeline. Then go to File>>Export and it will give you a bunch of options (email, web, etc) where it will show you the size of the movie after it is compressed (320×240, 160×120, etc.) as well as the file size (8MB, 4.2 MB, and so on. This way you don’t have to worry about the compression settings when exporting from Final Cut because Final Cut doesn’t give you an expected size of the file.
I don’t know how well an almost 5 minute clip will look at 320×240 at under 10MB, but this method is worth a shot. Good luck.