[marcoszorrilla]

procedure SetPaperSize (intAlto,intAncho : Integer);
{aqui se define el tamaño del papel}
var
   ADevice, ADriver, APort : array [0..255] of Char;
   ADeviceMode : THandle;
   DevMode : PDevMode;
begin
   with Printer do
      begin
         GetPrinter (ADevice, ADriver, APort, ADeviceMode);
         SetPrinter (ADevice, ADriver, APort, 0);
         GetPrinter (ADevice, ADriver, APort, ADeviceMode);
         DevMode := GlobalLock(ADeviceMode);
         if not Assigned(DevMode) then
            ShowMessage('Ha entrado en -> if not Assigned(DevMode) ...')
         else
            begin
               with DevMode^ do
                  begin
                     dmPaperSize:= DMPAPER_User;
                     dmPaperLength := intAlto;
                     dmPaperWidth:= intAncho;
                     dmFields := dmFields or DM_PAPERSIZE or DM_PAPERLength or DM_PAPERWidth;
                  end;
               GlobalUnLock(ADeviceMode);
               SetPrinter(ADevice, ADriver, APort, ADeviceMode);
           end;
      end;
end;

Un Saludo.