Hi,
Thanks for your interest in Syncfusion Products.
As far as now we don't provide option for retrieving the radius. If your motive is to make the Pie occupy the full area, you could use a DataTemplate, that will scale the segments.
Code Snippet:
Data="{Binding Geometry}" ToolTip="{Binding ToolTip}">
Please refer to the sample attached in the following link.
http://files.syncfusion.com/support/Chart.WPF/7.3.0.6/F82601/main.htmPlease let us know if you have further queries.
Regards,
Sowmya.N