If you resize the Auto-Tune Pro window via the Preferences menu while in the Auto mode, a black border will appear around the Auto mode interface. This is the expected behavior because the Auto mode interface is not designed to stretch to fill the screen.
If you switch to the Graph mode after increasing the window size, you'll see that the Graph mode interface does stretch to fill the screen. An increased screen size may only be necessary if you're performing detailed pitch and time correction edits within Auto-Tune Pro's graphical editor, so this is why the Graph mode interface can resize to fill a larger window size.
If you don't like to see the black border around the Auto mode interface when you're working, click on the Settings drop-down menu, go to Preferences, click on the "Set Min Size" button or type in the minimum screen resolution values, and click "Save" to resize the window to the smallest setting.