Either your TV is overscanning (or underscanning), you don't have a 16:9 TV (it might be 16:10 which is PC ratio but seems unlikely for a year old DLP) or perhaps you have not done the screen size calibration under settings (it should have run the first time you installed GT5).
If your picture is too big for the screen, I would suggest first trying to turn off overscan, if that doens't work, run the screen size calibration tool in settings and zoom it down until it fits.
If your screen is too small, also try the screen size calibration too, but if it's only too small one way, it's almost certainly somemthing with your TV. See if your TV has any zoom controls and try to increase the height or decrease the width (then increase zoom) to make it fit.