Re-reading your post, I notice that you are saving the iMovie project to a USB drive (either a flash drive or hard disk?). My response above relates to creating a DVD that will be playable on a DVD Player. From your question, I assume that you are able to connect a USB drive directly to the DVD Player (rather than playing a DVD).
If you are putting the movie on a USB drive you still need to Share the movie. As well as Share>Media Browser, you have other Share options, such as Share>Export Movie and Share>Export using QuickTime. Using Share>Export Movie, you can choose different size options and also choose where you wish to save the movie, for example, Desktop, Movies folder or other location on your hard drive or external drive. You could, for example, save it directly to your USB flash drive or external hard drive.
John