public Nullable<double> LowerSizeStop {get; set;}

string colorBrewerSchemesName = "ArcGIS Colors";
StyleProjectItem style = Project.Current.GetItems<StyleProjectItem>().First(s => s.Name == colorBrewerSchemesName);
string colorRampName = "Heat Map 4 - Semitransparent";
IList<ColorRampStyleItem> colorRampList = await QueuedTask.Run(() =>
{
  return style.SearchColorRamps(colorRampName);
});
ColorRampStyleItem colorRamp = colorRampList[0];

await QueuedTask.Run(() =>
{
  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
  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
  };
  FeatureLayer flyr = MapView.Active.Map.Layers[0] as FeatureLayer;
  CIMProportionalRenderer propRndr = flyr.CreateRenderer(prDef) as CIMProportionalRenderer;
  flyr.SetRenderer(propRndr);

});