C#

private void pd_PrintPage(object sender, PrintPageEventArgs e)
{
  // I don't rem
  e.PageSettings.PaperSize =
    new PaperSize("Custom", 11.69 * 100, 16.54 * 100);
}

foreach (PaperSize size in PrinterSettings.PaperSizes)
  if (size.Kind == PaperKind)
    return size;
return null; // or throw an Exception that A3 is not supported

private void MyButtonPrint_Click(object sender, System.EventArgs e)
   {
      // Set the paper size based upon the selection in the combo box.
      if (comboPaperSize.SelectedIndex != -1) {
         printDoc.DefaultPageSettings.PaperSize = 
            printDoc.PrinterSettings.PaperSizes[comboPaperSize.SelectedIndex];
      }

      // Set the paper source based upon the selection in the combo box.
      if (comboPaperSource.SelectedIndex != -1) {
         printDoc.DefaultPageSettings.PaperSource = 
            printDoc.PrinterSettings.PaperSources[comboPaperSource.SelectedIndex];
      }
      
      // Set the printer resolution based upon the selection in the combo box.
      if (comboPrintResolution.SelectedIndex != -1) 
      {
         printDoc.DefaultPageSettings.PrinterResolution= 
            printDoc.PrinterSettings.PrinterResolutions[comboPrintResolution.SelectedIndex];
      }

      // Print the document with the specified paper size, source, and print resolution.
      printDoc.Print();
   }

[DllImport("whatever.dll")]
private int testfunc(ref IntPtr struct);