Resolution to "Streched"?

Hi! I just installed Windows via bootcamp on a partion, mainly for CS:GO, because it runs smoother on windows. But I have one problem. I cant strech the resolution of the game (when i play 4:3 I want to strech it to the edges), in Mac OS when I choose 4:3 I cant choose anything but streched...

I hvae watched videos of how to change the resolution (windows). And there two ways, either do it on your monitor (where iMac dont have any buttons/options), or do it in the graphics card. The problem is I cant access the AMD Catalyst center, so I cant change the resolution... Is there any ways of doing this? Accessing AMD catalyst? Or something to strech the resolution? Please help!

/Rasmus Lundberg