Sometimes the taskbar covers a part of your application window and you would like to avoid that.
The following procedure shows how to retrieve the size of the taskbar ('tray window');
the key information is the internal window's classname Shell_TrayWnd:
procedure TForm1.Button1Click(Sender: TObject);
var
Tasklist : HWnd;
Bordered : TRect;
begin
Tasklist := FindWindow('Shell_TrayWnd', nil);
GetWindowRect(Tasklist, Bordered);
Label1.Caption := 'Left: ' + IntToStr(Bordered.Left);
Label2.Caption := 'Right: ' + IntToStr(Bordered.Right);
Label3.Caption := 'Top: ' + IntToStr(Bordered.Top);
Label4.Caption := 'Bottom: ' + IntToStr(Bordered.Bottom);
end;