public double Size {get; set;}

//var options = ApplicationOptions.EditingOptions;
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);
          }
});