public static void SetSize( 
   CIMSymbol symbol,
   double size
)

{
  //Note: Must be on QueuedTask.Run

  //Build geometry
  List<Coordinate2D> plCoords = new List<Coordinate2D>();
  plCoords.Add(new Coordinate2D(1, 8.5));
  plCoords.Add(new Coordinate2D(1.66, 9));
  plCoords.Add(new Coordinate2D(2.33, 8.1));
  plCoords.Add(new Coordinate2D(3, 8.5));
  Polyline linePl = PolylineBuilderEx.CreatePolyline(plCoords);

  //Reference a line symbol in a style
  var ProjectStyles = Project.Current.GetItems<StyleProjectItem>();
  StyleProjectItem style = ProjectStyles.First(x => x.Name == "ArcGIS 2D");
  var symStyle = style.SearchSymbols(StyleItemType.LineSymbol, "Line with 2 Markers")[0];
  CIMLineSymbol lineSym = symStyle.Symbol as CIMLineSymbol;
  lineSym.SetSize(20);

  //Set symbology, create and add element to layout
  //CIMLineSymbol lineSym = SymbolFactory.Instance.ConstructLineSymbol(ColorFactory.Instance.BlueRGB, 4.0, SimpleLineStyle.Solid);
  ElementFactory.Instance.CreateGraphicElement(
    layout, linePl, lineSym, "New Line");
}

{
  //Note: Must be on QueuedTask.Run(() => { ...

  //Build geometry
  Coordinate2D coord2D = new Coordinate2D(2.0, 10.0);

  //Reference a point symbol in a style
  StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>()
            .FirstOrDefault(item => item.Name == "ArcGIS 2D");
  SymbolStyleItem symStyleItm = stylePrjItm.SearchSymbols(
                        StyleItemType.PointSymbol, "City Hall")[0];
  CIMPointSymbol pointSym = symStyleItm.Symbol as CIMPointSymbol;
  pointSym.SetSize(50);

  var elemInfo = new ElementInfo()
  {
    CustomProperties = new List<CIMStringMap>() {
    new CIMStringMap() { Key = "Key1", Value = "Value1"},
    new CIMStringMap() { Key = "Key2", Value = "Value2"}
  },
    Anchor = Anchor.TopRightCorner,
    Rotation = 45.0
  };

  var graphic = GraphicFactory.Instance.CreateSimpleGraphic(
                                coord2D.ToMapPoint(), pointSym);

  ElementFactory.Instance.CreateGraphicElement(
    layout, graphic, "New Point", true, elemInfo);
}

{
  //Create a simple 2D point graphic and apply an existing point style item as the symbology.
  //Note: Must be on QueuedTask.Run
  //Build 2D point geometry  
  Coordinate2D coord2D = new Coordinate2D(2.0, 10.0);

  //(optionally) Reference a point symbol in a style
  StyleProjectItem ptStylePrjItm = Project.Current.GetItems<StyleProjectItem>()
                                .FirstOrDefault(item => item.Name == "ArcGIS 2D");
  SymbolStyleItem ptSymStyleItm = ptStylePrjItm.SearchSymbols(
                                         StyleItemType.PointSymbol, "City Hall")[0];
  CIMPointSymbol pointSym = ptSymStyleItm.Symbol as CIMPointSymbol;
  pointSym.SetSize(50);

  //Set symbology, create and add element to layout

  //An alternative simple symbol is also commented out below.
  //This would eliminate the four optional lines of code above that
  //reference a style.
  //CIMPointSymbol pointSym = SymbolFactory.Instance.ConstructPointSymbol(
  //                  ColorFactory.Instance.RedRGB, 25.0, SimpleMarkerStyle.Star);  

  GraphicElement ptElm = ElementFactory.Instance.CreateGraphicElement(
                                 layout, coord2D.ToMapPoint(), pointSym);
  ptElm.SetName("New Point");
}

{
  //Create a simple 2D line graphic and apply an existing line
  //style item as the symbology.
  //Note: Must be on QueuedTask.Run
  //Build 2d line geometry
  List<Coordinate2D> plCoords = new List<Coordinate2D>();
  plCoords.Add(new Coordinate2D(1, 8.5));
  plCoords.Add(new Coordinate2D(1.66, 9));
  plCoords.Add(new Coordinate2D(2.33, 8.1));
  plCoords.Add(new Coordinate2D(3, 8.5));

  Polyline linePl = PolylineBuilderEx.CreatePolyline(plCoords);

  //(optionally) Reference a line symbol in a style
  StyleProjectItem lnStylePrjItm = Project.Current.GetItems<StyleProjectItem>()
                                .FirstOrDefault(item => item.Name == "ArcGIS 2D");
  SymbolStyleItem lnSymStyleItm = lnStylePrjItm.SearchSymbols(
                                StyleItemType.LineSymbol, "Line with 2 Markers")[0];
  CIMLineSymbol lineSym = lnSymStyleItm.Symbol as CIMLineSymbol;
  lineSym.SetSize(20);

  //Set symbology, create and add element to layout

  //An alternative simple symbol is also commented out below.
  //This would eliminate the four optional lines of code above that
  //reference a style.
  //
  //CIMLineSymbol lineSym = SymbolFactory.Instance.ConstructLineSymbol(
  //         ColorFactory.Instance.BlueRGB, 4.0, SimpleLineStyle.Solid);  


  GraphicElement lineElm = ElementFactory.Instance.CreateGraphicElement(
                                              layout, linePl, lineSym);
  lineElm.SetName("New Line");
}