The elements are derived from Printer settings. You can change the paper size at runtime to defined sizes.
example snippet:
boReportDocument.Load(Server.MapPath("CrystalReport.rpt"));
PrintOptions boPrintOptions = boReportDocument.PrintOptions;
boPrintOptions.PaperSize = PaperSize.PaperA3;
boPrintOptions.PaperSize = PaperSize.PaperA4;
Try out!