Play around with the overscan adjustments, often labeled HSIZ and VSIZ. Use the HPOS and VPOS adjustments if the picture is not centered.
Some sources may fail to use all of the 1280 x 720 pixels in the video frame. If you optimize the picture for sources that do use the entire frame, then black edges will intrude on programs that do not use the entire frame. Also on some CRT RPTV's and direct view sets the picture shrinks slightly when the overall picture content is quite dark.
Video hints:
http://members.aol.com/ajaynejr/video.htm