With any object selected, clicking on the  FULL FRAME button will resize the object (or objects) to fill the screen (i.e. if your page setup is 7.5" x 10" then the objects selected will become 7.5" x 10" and be centered in the frame.

example:
For those who insert pictures floating on the slide for use as backgrounds, you must be tired of manually resizing them each time, just one click and they are sized to fill the frame.