The final size of the countertop is determined once a countertop has overhangs added and the corners are filleted. Once the final size is determined, backsplashes can be added.
Adding a Backsplash
Click Draw > Splash/Miter
Select Edge as theModeif it is not already selected.
Enter the Height > OK
Enter the Offset > OK
Click the outside of the edge to create the backsplash from.