You want to shrink the image to only fill the TV safe zone? Or, your clips is a lot smaller than TV safe, and you want to make it bigger?
Your footage should really fill the screen, out to the sides and corners. The TV safe zone really is only for old tube TVs, because HDTVs TV safe zone is much closer to the edge. And what it is used for is to make sure that the action you want to be seen falls within the area that most people can see. But really, your image should fill the screen.
Why doesn't it normally? If it doesn't, it means you aren't using sequence settings that match your footage settings.
But if for some reason you do need to resize or stretch the video like for instance if your video is the wrong size due to mixing multiple formats in one sequence;
Then the easiest way is to double click on the video when it's on the timeline and then select the 'motion' tab in the 'viewer' window. Then play with the 'Basic motion' (scale) and 'distort' tools (aspect ratio) until it looks the way you like.