I always have about 2 inches of black at the top and bottom of my screen no matter what I change the resolution to.
If you have a full screen monitor (not widescreen) look for 1024x768 or 1280x1024 or 1600x1200.
Are you changing resolution in the game menu or in Windows desktop?
DirectX will change whatever the Windows desktop is set to to what the game is set for.
It's possible some download versions of the game don't offer a choice between full screen and widescreen. But there are full screen images at Mobygames, so if you have a disc version you should be able to get a full screen picture.