When watching a pre-recording on our Panasonic TV the picture is reduced on the screen, with a black field on either side. How can I switch to a full screen

Press the screen mode on your remote. It should give you several choices.

Press the screen mode on your remote. It should give you several choices.

On the Panasonics, it is usually labeled "Format".

Keep in mind, that usually this stretches the picture or cops the picture to make it fit on the widescreen TV. The bars on the side are usually showing the programming as intended.