First, you must set the WindowState property of the form to Minimized. Then you must create a message handler to respond to the Windows WM_QueryOpen message, to let Windows know that you do not want the form to resize. Example:

{Place this code in the private section of the Form declaration}
procedure WMQueryOpen(VAR Msg : TWMQueryOpen); message WM_QUERYOPEN;
{Place this code in the Form implementation section}
procedure TForm .WMQueryOpen(VAR Msg : TWMQueryOpen);
begin
  Msg.Result := 0;
end;