If the box is connected to the tv with hdmi
and there is a Comcast remote also then the size and shape of the picture on your screen is set in two places. One is the Tv the other is the comcast box. You need to experiment using a combination of BOTH of these settings until you find a size/shape you like.
You already found the button on the Panasonic remote.
Now find the button on your comcast box remote the button may be called something like- HD Zoom, Aspect, Format...
Pressing those button repeatedly will cycle to the next available size/shape which may vary depending on if you are looking at an HD channel or SD channel. Again use BOTH remote to find a size/shape you like.
If you need more help, reply back here.
Before you go, please be sure to take a moment right now to click the positive rating so that I get credit for helping you today. Doing so does not close the question or prevent us from continuing our conversation here. My livelihood truly depends on your positive rating.
Please do not click bad-poor rating if you are unhappy. Doing that destroys my standing here as a professional and does not issue you a refund. Instead REPLY back here.