The size of the screen doesn't matter Gary, it's the shape that determines whether or not the entire screen will be filled with the image (at its correct aspect ratio) in the game window.
Imagine that the 3 images below are actual monitors, the first is a 4:3 aspect ratio screen, the same as the game...

...the image will fill the entire screen.
The next is a widescreen monitor 16:9 (or 16:10, which is what I have but they're not widely available now apparently)...

...because the screen is wider than the game image it won't fill the screen's width.
And if they produced screens like this or you have software installed on your PC to rotate the screen image...

...then the game image would expand to fit the width but not the height.