This setting determines how much of the available visual container area is used to draw the donut chart. The default value is 60%, but you can use a custom percent value between 20 and 100.

Smaller radius values will reduce the donut radius and create more blank space, while higher values will create a larger donut chart within the available container. 

Keep in mind that a large donut chart radius value may not leave enough space for Detail Labels (with Outside asLabel Placement) or Legend to be displayed properly.