The problem with HDMI is that it does not always scale properly.
If you cannot find the Catalyst Control Center (CCC), then either download and use Driver Sweeper, or uninstall CCC from the control panel yourself. And then re-install it. Are you sure you installed it in there first place?
I'm not in front of my PC right now, but the scaling option should be in the DVI section of CCC. I have not used CCC in a while so I am not exactly sure where the scaling option is. It seems it is under scanning so you need to move the slider towards over scan.