uses CommCtrl;

procedure SetTreeViewItemHeight(aTreeView: TTreeView; aItemHeight: Word);
begin
  aTreeView.Perform(TVM_SETITEMHEIGHT, aItemHeight, 0);
end;

// Пример:

procedure TForm1.Button1Click(Sender: TObject);
begin
  SetTreeViewItemHeight(TreeView1, 30);
end;