Choose a screen size
You can set a movie to play at the size that best fits how you want to watch it.
After you open the movie, choose a size option from the View menu:
Enter Full Screen: The movie is displayed as large as possible and the menu bars are not visible. To exit full-screen view, press Escape.
You can also click the Full Screen button
in the upper-left corner to enter full-screen view. To leave full-screen view, move the pointer to the upper-left corner of the screen, then click the Exit Full Screen button 
. For more information, see Use apps in full screen or Split View.Actual Size: The movie is displayed at its source size.
Fit to Screen: The movie is scaled to fit your screen.
Fill Screen (only in full-screen view): The movie is scaled and cropped to completely fill your screen in at least one dimension. This can eliminate black areas on the top (letter boxing) or side (pillar boxing), but may hide or distort the content.
Panoramic (only in full-screen view): The movie is scaled to fit your screen, and the outer horizontal edges are compressed to avoid cropping the image.
To increase or decrease the movie window size from any size option, choose View > Increase Size or View > Decrease Size.
If you want the file you’re playing to appear in front of all other onscreen windows, choose View > Float on Top. When this setting is turned on, a checkmark appears next to Float on Top. Choose it again to turn it off.