Wacom Intuos Pro Manual page 39

Tablet Area Mapping
Use the Tablet Area option of the Mapping tab to define the tablet area that will be mapped to the screen
area.
Full maps the entire active area of the tablet. This is the default setting.
Portion... In the Portion of Tablet dialog, choose a method for defining a portion of the display:
Drag the corners of the foreground graphic to select the screen area.
Select the Click to Define Tablet Area button and use your tool on the tablet to select the area.
Be sure to follow any prompts.
Enter coordinate values.
The above screenshot is for descriptive purposes; it may not match your screen.