If you have a second display connected to your computer, you can use it to play video. When you do so, the iMovie viewer appears on the second display only, thereby expanding your workspace in the iMovie window.
To set iMovie to show video on a second display:
Connect a second display to your computer.
For instructions about how to connect a second display, choose Help > Mac Help and enter “connecting multiple displays” in the search field. To view the Help menu, click your desktop so that the “Finder” is active (you’ll see the word Finder in the upper-left corner of your screen), and then click Help in the light gray bar at the top of your screen.
Make sure Advanced Tools are turned on in iMovie preferences.
In iMovie, choose Window > “Viewer on Other Display.”
The Window menu appears in a light gray bar across the top of your computer screen.
To return the viewer to the iMovie window, choose Window, and then deselect “Viewer on Other Display.”