public class CIMPage : CIMObject, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

//Modify a layouts page settings.

//Perform on the worker thread
await QueuedTask.Run(() =>
{

  CIMPage page = layout.GetPage();

  //Do something 

  layout.SetPage(page);
});

//This example creates a new layout using a CIM page definion with rulers and guides included.

//Added references
using ArcGIS.Desktop.Layouts;                      
using ArcGIS.Desktop.Framework.Threading.Tasks;    
using ArcGIS.Desktop.Core;
using ArcGIS.Core.CIM;

public class CreateLayoutEx2
{
  async public static Task<Layout> CreateCIMLayout(double width, double height, LinearUnit units, string LayoutName)
  {
    Layout CIMlayout = null;
    await QueuedTask.Run(() =>
    {
      //Set up a page
      CIMPage newPage = new CIMPage();

      //required
      newPage.Width = width;
      newPage.Height = height;
      newPage.Units = units;

      //optional rulers
      newPage.ShowRulers = true;
      newPage.SmallestRulerDivision = 5;

      //optional guides
      newPage.ShowGuides = true;
      CIMGuide guide1 = new CIMGuide();
      guide1.Position = 25;
      guide1.Orientation = Orientation.Vertical;
      CIMGuide guide2 = new CIMGuide();
      guide2.Position = 185;
      guide2.Orientation = Orientation.Vertical;
      CIMGuide guide3 = new CIMGuide();
      guide3.Position = 25;
      guide3.Orientation = Orientation.Horizontal;
      CIMGuide guide4 = new CIMGuide();
      guide4.Position = 272;
      guide4.Orientation = Orientation.Horizontal;

      List<CIMGuide> guideList = new List<CIMGuide>();
      guideList.Add(guide1);
      guideList.Add(guide2);
      guideList.Add(guide3);
      guideList.Add(guide4);
      newPage.Guides = guideList.ToArray();

      Layout layout = LayoutFactory.Instance.CreateLayout(newPage);

      layout.SetName(LayoutName);
    });

    //Open the layout in a pane on the UI!
    await ProApp.Panes.CreateLayoutPaneAsync(CIMlayout);
    return CIMlayout;
  }
}

//Create a new layout using a modified CIM and open it.
//The CIM exposes additional members that may not be
//available through the managed API.  
//In this example, optional guides are added.

//Create a new CIMLayout on the worker thread
Layout newCIMLayout = await QueuedTask.Run(() =>
{
  //Set up a CIM page
  CIMPage newPage = new CIMPage
  {
    //required parameters
    Width = 8.5,
    Height = 11,
    Units = LinearUnit.Inches,

    //optional rulers
    ShowRulers = true,
    SmallestRulerDivision = 0.5,

    //optional guides
    ShowGuides = true
  };
  CIMGuide guide1 = new CIMGuide
  {
    Position = 1,
    Orientation = Orientation.Vertical
  };
  CIMGuide guide2 = new CIMGuide
  {
    Position = 6.5,
    Orientation = Orientation.Vertical
  };
  CIMGuide guide3 = new CIMGuide
  {
    Position = 1,
    Orientation = Orientation.Horizontal
  };
  CIMGuide guide4 = new CIMGuide
  {
    Position = 10,
    Orientation = Orientation.Horizontal
  };

  List<CIMGuide> guideList = new List<CIMGuide>
  {
    guide1,
    guide2,
    guide3,
    guide4
  };
  newPage.Guides = guideList.ToArray();

  //Construct the new layout using the customized cim definitions
  var layout_local = LayoutFactory.Instance.CreateLayout(newPage);
  layout_local.SetName("New 8.5x11 Layout");
  return layout_local;
});

//Open new layout on the GUI thread
await ProApp.Panes.CreateLayoutPaneAsync(newCIMLayout);

//Change the layout page size.

//Reference the layout project item
LayoutProjectItem lytItem = Project.Current.GetItems<LayoutProjectItem>()
                         .FirstOrDefault(item => item.Name.Equals("MyLayout"));
if (layoutItem != null)
{
  await QueuedTask.Run(() =>
  {
    //Get the layout
    Layout lyt = lytItem.GetLayout();
    if (lyt != null)
    {
      //Change properties
      CIMPage page = lyt.GetPage();
      page.Width = 8.5;
      page.Height = 11;

      //Apply the changes to the layout
      lyt.SetPage(page);
    }
  });
}

//Note: Call within QueuedTask.Run()
//The fields in the datasource used for the report
//This uses a US Cities dataset
var listFields = new List<CIMReportField> {
          //Grouping should be the first field
          new CIMReportField{Name = "STATE_NAME", FieldOrder = 0, Group = true, SortInfo = FieldSortInfo.Desc}, //Group cities using STATES
          new CIMReportField{Name = "CITY_NAME", FieldOrder = 1},
          new CIMReportField{Name = "POP1990", FieldOrder = 2, },
      };
//Definition query to use for the data source
var defQuery = "STATE_NAME LIKE 'C%'";
//Define the Datasource
var reportDataSource = new ReportDataSource(featureLayer, defQuery, listFields);
//The CIMPage defintion - page size, units, etc
var cimReportPage = new CIMPage
{
  Height = 11,
  StretchElements = false,
  Width = 6.5,
  ShowRulers = true,
  ShowGuides = true,
  Margin = new CIMMargin { Bottom = 1, Left = 1, Right = 1, Top = 1 },
  Units = LinearUnit.Inches
};

//Report template
var reportTemplates = await ReportTemplateManager.GetTemplatesAsync();
var reportTemplate = reportTemplates.Where(r => r.Name == "Attribute List with Grouping").First();

//Report Styling
var reportStyles = await ReportStylingManager.GetStylingsAsync();
var reportStyle = reportStyles.Where(s => s == "Cool Tones").First();

//Field Statistics
var fieldStatisticsList = new List<ReportFieldStatistic> {
          new ReportFieldStatistic{ Field = "POP1990", Statistic = FieldStatisticsFlag.Sum}
          //Note: NoStatistics option for FieldStatisticsFlag is not supported.
      };
var report = ReportFactory.Instance.CreateReport("USAReport", reportDataSource, cimReportPage, fieldStatisticsList, reportTemplate, reportStyle);

//Note: Call within QueuedTask.Run()
var cimReportPage = new CIMPage
{
  Height = 12,
  StretchElements = false,
  Width = 6.5,
  ShowRulers = true,
  ShowGuides = true,
  Margin = new CIMMargin { Bottom = 1, Left = 1, Right = 1, Top = 1 },
  Units = LinearUnit.Inches
};
report.SetPage(cimReportPage);
//Change only the report's page height
report.SetPageHeight(12);