This problem with the picture size could be coming from either the tv OR the box. It is more likely the box than the tv.
Grab the cable box remote and look for a button that says FORMAT, 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.
Let me know if you get stuck.