True "full screen" mode expands the window to the entire screen without toolbars and menus impeding your view. Often, people describe a maximized window as full screen, although it is technically a different mode, because menus and toolbars remain visible. Still, this mode does expand the window across the entire screen, so it is effectively full screen. Shrinking either full screen mode requires putting it in normal windowed mode where dragging the borders allow you to resize the window.
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Press "F11," "Esc" or the a combination of "Alt" and "Enter" to exit true full screen mode. The window then returns to maximize or normal windowed mode.
Press the Restore Down button, which is the double-window icon located immediately to the left of the corner "X" button. This places the window in normal windowed mode. If this icon displays only a single square, then the window is already in normal windowed mode.
Point your mouse pointer over any corner until it turns into a diagonal double-arrow.
Click and drag the corner until the window is the desired size. Release your mouse button to accept the new size.