This is annoying as ***, and I'm pretty sure it's an oversight in the mini box firmware that Cox won't bother fixing. I have a few old 4:3 CRTs around the house, and the picture is completely cropped off on the sides. The worst part is that this doesn't seem to be adjustable in the settings either. If I go to Settings->Audio & Video->Video Output Form, the only option is for "480i standard" resolution. According to the documentation, I should be able to hit right-arrow and choose the aspect ratio, but pressing the right arrow doesn't bring up any options for changing the aspect ratio.
I can sort of fix this by pressing the "zoom" button on the remote a few times. This gets the picture "unzoomed" to a somewhat reasonable letterbox, but the sides are still cropped a little bit. This is more obvious on channels like ESPN that have graphics near the edge of the screen.
I doubt Cox is even going to bother fixing this since they seem to think that nobody uses CRTs any more, but it really wouldn't be that hard to fix. Makes me wonder why I even bother to keep this service.