
type
  TMyDBGrid = class(TDBGrid)
  public
    property DefaultRowHeight;
  end;
var
  Form1: TForm1;
implementation
{$R *.DFM}

procedure TForm1.FormCreate(Sender: TObject);
begin
  DbGrid1.Canvas.Font.Size := 13;
  TMyDBGrid(DBGrid1).DefaultRowHeight := DBGrid1.Canvas.TextHeight('MMMMM') + 4;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  DBGrid1.DataSource.DataSet.DisableControls;
  TStringGrid(DBGrid1).DefaultRowHeight := 55;
  DBGrid1.DataSource.DataSet.EnableControls;
end; 