i will assume you have a 1080p television here... (720p is similar in effect).
the ONLY content which will fill the screen is either 1080p or 1080i content or content which is the same aspect ratio. all other content will have black bars or appear correct in pixel by pixel size but be very small on screen.
your television should have different screen modes to swap through depending on what type of content you are watching. one should be fullscreen which fits the content to the screen. some non 1080p content will have black bars. one is stretch which will take a squareish image and widen it to fill the screen but will distort the image. one views content at its native size so any content (such as 480p) will appear sharper but will only take up about half the tv. last there should be a zoom which enlarges everything by a percentage but will cut off the top and bottom of older content normally.
besides these (and similar) modes there isnt much you can do. well....except use only 1080p or 720p content.