uses
  CommCtrl;

procedure TForm1.Button1Click(Sender: TObject);
var
  ItemEx: TTVItemEx;
  InsertStruct: TTVInsertStruct;
begin
  ItemEx.mask := TVIF_TEXT or TVIF_INTEGRAL;
  ItemEx.pszText := PChar('This node is 3 times higher than default node height');
  // this is the multiplier of default node height, which you can set by sending
  // TVM_SETITEMHEIGHT message even to 1 pixel, but then the lines become broken
  // so if it's sufficient for you to use multiples of default node height, this
  // could be the way but it's "quite difficult" to modify the original VCL tree
  // view control to use the TTVItemEx structure for nodes
  ItemEx.iIntegral := 3;

  InsertStruct.hParent := nil;
  InsertStruct.hInsertAfter := TVI_LAST;
  InsertStruct.itemex := ItemEx;

  TreeView_InsertItem(TreeView1.Handle, InsertStruct);
end;