Step 1: First of all you will want to make sure that you’re selected on the timeline.
Step 2: Then go to View, Video Playback, and click on your designated display, (either a camera, internal or external display).
Step 3: Next, go back up to View, External Video, and click on All Frames (⌘F12). That’s it. You should now automatically be launched into fullscreen mode. To get out of fullscreen, just press the escape key.
Source: Mac Filmmaking