Here are ways to prepare your iMovie for iDVD:
In iMovie 8.0 or later, choose Share > iDVD. Your movie is rendered efficiently and sent to iDVD.
If you want to add your video project to an iDVD project already in progress, send it to the Media Browser and select the largest movie size available. iMovie renders your project and then makes it available to all Apple applications that can access the Media Browser, including iDVD, iWeb, and GarageBand.
In iMovie, choose Share > Media Browser.
Choose which size of movie to share, and then click Publish. Large is the suggested size for use in iDVD. If your camcorder does not shoot high-definition video, however, the large size option may not be available.
When you share your movie, a self-contained QuickTime movie appears automatically in the Movies section of the Media pane of iDVD. You can then add the movie to any iDVD project.
In iMovie, choose Share > Export Movie.
Choose which size of movie to export. Large is the suggested size for use in iDVD. Choose where to save the movie, and then click Export. If you save the movie to your Movies folder, it appears automatically in the Movies folder in the Movies pane of iDVD.