My iMovie trailer clip is too short. Is there a way to make the clip longer?  I thought of maybe looping it or duplicating it but I can't figure out how.  The slow motion option won't work either.  Do the presets in the movie trailers stop you from doing this?
Looping is a function of the player (i.e., QuickTime). You can copy the clip and paste into the timeline of an iMovie project several times to create a loop effect if you want, but it will only play as many times as the clip is copied into the iMovie project.