Choose a screen size in QuickTime Player on Mac
You can set a movie to play at the size that best fits how you want to watch it.
In the QuickTime Player app on your Mac, choose any of the following size options 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 the Esc (Escape) key.
You can also move the pointer to the green button in the top-left corner of the window, then choose Enter Full Screen from the menu that appears, or click the
. To leave the full-screen view, move the pointer to the green button again, then choose Exit Full Screen from the menu that appears, or click the button 
. See Use apps in full screen and Use apps in Split View.Actual Size: The movie is displayed at its source size.
Fit to Screen: The movie is scaled to fit your screen.
Full 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 so a checkmark appears next to it. Choose it again to turn it off.