Hi, this sounds very similar to a problem that I had when I first connected my tv to my computer via hdmi. If it is the same problem I had, all you have to do is go to the catalyst control center. From there click desktops and displays, then my digital flat panels, then scaling options. Once there, move the slider to 0% and the picture should now fill your screen. I hope this helps.
This solved my problem. Thanks a lot