This problem with the picture size could be coming from either the tv OR the box
Grab the cable box remote and look for a button that says ZOOM, ASPECT, WIDE, RATIO, SIZE, etc
You may need to press this several times to cycle through the different options,
and you may have to make a similar adjustment on the tv as well, until you get them both to a desired setting
typically you'd want the cable box set on 16:9 or Wide
and you'd want the tv set on Normal or 16:9
It can take a bit of fiddling to get them both matched up, but you should have a button for this function on both the tv remote and on the box remote, and will have to use both to find the right balance.