Determines how the size of the overall pie chart symbol is set. Size can be either fixed: PieChartSizeOptions.Fixed, default, (all the same size) or proportional. If it is proportional, it is based either on the value of a particular field: PieChartSizeOptions.Field or the sum of all (eligible) fields: PieChartSizeOptions.SumSelectedFields
For a value of PieChartSizeOptions.Field, specifiy the name of the field to use in FieldName otherwise the first eligible field will be used.

Reference

PieChartRendererDefinition Class
PieChartRendererDefinition Members