public double Size {get; set;}

{
  var options = ApplicationOptions.EditingOptions;
  //var options = ApplicationOptions.EditingOptions;
  await QueuedTask.Run(() =>
  {
    //change the regular unselected vertex symbol
    //default is a green, hollow, square, 5pts. Change to
    //Blue outline diamond, 10 pts
    var vertexSymbol = new VertexSymbolOptions(VertexSymbolType.RegularUnselected);
    vertexSymbol.OutlineColor = ColorFactory.Instance.BlueRGB;
    vertexSymbol.MarkerType = VertexMarkerType.Diamond;
    vertexSymbol.Size = 10;

    //Are these valid?
    if (options.CanSetVertexSymbolOptions(
         VertexSymbolType.RegularUnselected, vertexSymbol))
    {
      //apply them
      options.SetVertexSymbolOptions(VertexSymbolType.RegularUnselected, vertexSymbol);
    }
  });
}