public class ProportionalRendererDefinition : RendererDefinition 

{
    string colorBrewerSchemesName = "ArcGIS Colors";
    //Get the style project item that contains the color ramps
    //Refer to the Initialize region for an example of how to get a style item
    //Note: Run within a QueuedTask
    //Creating a point symbol to be used as a template symbol for the proportional renderer
    CIMPointSymbol pointSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.GreenRGB, 1.0, SimpleMarkerStyle.Circle);
    CIMSymbolReference symbolPointTemplate = pointSym.MakeSymbolReference();

    //minimum symbol size is capped to 4 point while the maximum symbol size is set to 50 point
    //Creating a proportional renderer definition that uses the "Population" field
    ProportionalRendererDefinition prDef = new ProportionalRendererDefinition("POPULATION", symbolPointTemplate, 4, 50, true)
    {
        //setting upper and lower size stops to stop symbols growing or shrinking beyond those thresholds
        UpperSizeStop = 5000000,  //features with values >= 5,000,000 will be drawn with maximum symbol size
        LowerSizeStop = 50000    //features with values <= 50,000 will be drawn with minimum symbol size
    };
    // Create a proportional renderer using the definition
    CIMProportionalRenderer propRndr = featureLayer.CreateRenderer(prDef) as CIMProportionalRenderer;
    // Set the renderer to the feature layer
    featureLayer.SetRenderer(propRndr);
}

{
    string colorBrewerSchemesName = "ArcGIS Colors";
    //Get the style project item that contains the color ramps
    //Refer to the Initialize region for an example of how to get a style item
    //Note: Run within a QueuedTask
    //Creating a point symbol to be used as a template symbol for the proportional renderer
    CIMPointSymbol pointSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.GreenRGB, 1.0, SimpleMarkerStyle.Circle);
    CIMSymbolReference symbolPointTemplate = pointSym.MakeSymbolReference();

    //Defining proportional renderer where size of symbol will be same as its value in field used in the renderer.
    ProportionalRendererDefinition prDef = new ProportionalRendererDefinition("POPULATION", esriUnits.esriMeters, symbolPointTemplate, SymbolShapes.Square, ValueRepresentations.Radius);
    //Create a Proportional renderer using the definition
    CIMProportionalRenderer propRndr = featureLayer.CreateRenderer(prDef) as CIMProportionalRenderer;
    // Set the renderer to the feature layer
    featureLayer.SetRenderer(propRndr);
}

/// <summary>
/// Renders a feature layer using proportional symbols to draw quantities.
/// ![Proportional Symbols renderer](https://ArcGIS.github.io/arcgis-pro-sdk/images/Renderers/proportional-renderer.png)
/// </summary>
{
  //Check feature layer name
  //Code works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data
  var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.Name == "USDemographics");
  if (featureLayer == null)
  {
    MessageBox.Show("This renderer works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data", "Data missing");
  }
  //Gets the first numeric field of the feature layer
  var firstNumericFieldOfFeatureLayer = "NumericFieldInLayer";
  //Gets the min and max value of the field
  var sizes = GetFieldMinMax(featureLayer, firstNumericFieldOfFeatureLayer);
  ProportionalRendererDefinition prDef = new ProportionalRendererDefinition()
  {
    Field = firstNumericFieldOfFeatureLayer,
    MinimumSymbolSize = 4,
    MaximumSymbolSize = 50,
    LowerSizeStop = Convert.ToDouble(sizes.Item1),
    UpperSizeStop = Convert.ToDouble(sizes.Item2)
  };
  //Note: Run within QueuedTask
  CIMProportionalRenderer propRndr = (CIMProportionalRenderer)featureLayer.CreateRenderer(prDef);
  featureLayer.SetRenderer(propRndr);
}