You can work in iMovie using the full screen of your display to take advantage of every pixel it offers.
To use iMovie in Full Screen:
Click the Full Screen button in the upper-right corner of the iMovie window, or choose View > Enter Full Screen.
In Full Screen, the gray menu bar at the top of your screen disappears. Make it appear by moving your pointer to the top of the screen.
To exit Full Screen, click the Full Screen button in the upper-right corner of the menu bar, choose View > Exit Full Screen, or press Escape.
To learn how to play back your projects and Events so they fill the entire screen, instead of just the viewer, go to this topic: Play your video in full-screen playback mode.