Documentation

Outer Radius

This property allows you to set the Pie/Donut chart (outer) radius. The value in the setting is displayed in percentage.  Percent values are relative to the chart container area size.

By default, pie/donut outer radius is set to “60”

For the chart to be properly displayed the set value must be between "20 and 100".
power-bi-donut-chart-outer-radius-setting