Make Cover Photo Size Smaller

Hi... Can some tell how to change the photo cover size to 850x315 please? I think it's currently 1280x1280, there are premade cover photos i want to use for my site but they're to small and leaves white space around them, when i stretch them to fit they become pixelated. And if i use a wallpaper and shrink it to fit it doesn't look right, it looks smashed. So i just want to change the size to the common size I've been seeing.... Thank You