Barbara's settings will reset the tool to its default behavior.
More specifically, you probably have the Clipped option turned off. With the default settings, the clone overlay (which shows you a preview of what you are about to copy) is reduced to the size of your brush. With this option off, the entire image will be shown, centered on where you mouse currently is. You can turn this option back on by going to the Clone Overlay button in the Tool Options at the bottom and adding a check next to Clipped. The other posibility is that the Brush size for the tool is at least the same size as the entire image.
Resetting the tool will take care of both of these options.