Title: Change size of other window
Use FindWindow function to find handle of the foreign window and MoveWindow function to change position and dimensions of this window.
procedure TForm1.Button1Click(Sender: TObject);
var
MyHandle: THandle;
WinRect: TRect;
begin
MyHandle:=FindWindow(nil, 'Delphi Help');
GetWindowRect(MyHandle, WinRect);
MoveWindow(MyHandle, WinRect.Left, WinRect.Top, 300, 300, True);
end;