You can disable the viewport by clicking in the corner of the preview window where you’ll see a small arrow and some horizontal lines. Ensure ‘Show Aspect Frame’ is unchecked. The aspect frame is used to show what parts of the scene will actually be rendered.
As for the Reality window, what is your screen resolution? It might be a little too big to fit. I’m not sure how you’d go about fixing this unfortunately, as you can only resize the window from the bottom, and if that’s the missing section, it’s not going to be an easy task.