public void SetPage( 
   CIMPage page
)

//Modify a layouts page settings.

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

  CIMPage page = layout.GetPage();

  //Do something 

  layout.SetPage(page);
});

//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);
    }
  });
}