procedure TForm1.Button1Click(Sender: TObject);
var
  Device: array[0..255] of char;
  Driver: array[0..255] of char;
  Port: array[0..255] of char;
  hDMode: THandle;
  PDMode: PDEVMODE;
begin
  Printer.PrinterIndex := Printer.PrinterIndex;
  Printer.GetPrinter(Device, Driver, Port, hDMode);
  if hDMode <> 0 then begin
    pDMode := GlobalLock(hDMode);
    if pDMode <> nil then begin

     {Set to legal}
      pDMode^.dmFields := pDMode^.dmFields or dm_PaperSize;
      pDMode^.dmPaperSize := DMPAPER_LEGAL;

     {Set to custom size}
      pDMode^.dmFields := pDMode^.dmFields or
        DM_PAPERSIZE or
        DM_PAPERWIDTH or
        DM_PAPERLENGTH;
      pDMode^.dmPaperSize := DMPAPER_USER;
      pDMode^.dmPaperWidth := 100 {SomeValueInTenthsOfAMillimeter};
      pDMode^.dmPaperLength := 100 {SomeValueInTenthsOfAMillimeter};

     {Set the bin to use}
      pDMode^.dmFields := pDMode^.dmFields or DMBIN_MANUAL;
      pDMode^.dmDefaultSource := DMBIN_MANUAL;

      GlobalUnlock(hDMode);
    end;
  end;
  Printer.PrinterIndex := Printer.PrinterIndex;
  Printer.BeginDoc;
  Printer.Canvas.TextOut(100, 100, 'Test 1');
  Printer.EndDoc;
end;