Are you using a cable box? Which one? What channels are you having problems with? Do you have an HD TV? How is your TV connected to the cable? In what way does the picture not fit the TV screen?
If the problem is with HD channels, you may need to make adjustments on the TV. If it's with non-HD channels, you may have the picture zoomed. There are picture settings for the box you can access by turning off the box with the remote and then hitting the "Menu" button. Check your TV manual for TV adjustments.
Since they added the new TV Guide the menu button no longer allows user to zoom the picture size, so everything except high definition appears as a tiny picture. It's like having a 19-inch picture on my 25-inch screen. There are complex ways to stretch the picture, but that looks weird. Zoom is gone. If this is not fixed fast we will soon be switching our 4 TVs and 3 computers to Verizon FIOS. If Wall Street figures this out before comcast does, the smart guys will be shorting Comcast stock down, down, down.