If you like the movie trailer format but you want to customize the length of the clip segments, you can convert the special movie trailer project into a normal iMovie project by going to (File>Convert Trailer to Movie).
Video Tutorial about this: https://www.youtube.com/watch?v=-lBbqgqgTVA
Otherwise you need to edit your project manually. A trick to limit the length of a project is to first insert a blank image into your project and to make it the length you want your video to be. Then you can start adding clips to your project by dragging the clips over the blank image (if it doesn't cover the clip completely select the cutaway function). This method can help your project stay a certain time length.