[angvelem]

Для изменения высоты выделенного Item-а, этого достаточно:

Код:

procedure TForm1.ListBox1DrawItem(Control: TWinControl; Index: Integer;
  Rect: TRect; State: TOwnerDrawState);
var
  Selected : Boolean;
begin
  Selected := (State = [odSelected, odFocused]) or (State = [odSelected]);
  if Selected then
    SendMessage(ListBox1.Handle, LB_SETITEMHEIGHT, Index, 45)
  else
    SendMessage(ListBox1.Handle, LB_SETITEMHEIGHT, Index, 16);

  with ListBox1.Canvas do
  begin
    FillRect(Rect);
    if Selected then
    begin
      Font.Style := [fsBold];
      Font.Size := 24;
    end
    else
    begin
      Font.Style := [];
      Font.Size := 8;
    end;
    TextOut(Rect.Left + 1, Rect.Top + 1, ListBox1.Items[Index]);
  end;
end;