interface
uses ........................ , Message;

  private
    procedure WMGetMinMaxInfo(var MSG: Tmessage);message WM_GetMinMaxInfo;

implementation
..
..

//=========================================================
procedure TMDIChild.WMGetMinMaxInfo(var MSG:Tmessage);
begin
  inherited;
  with PMinMaxInfo(MSG.lparam)^ do
  begin
    with ptMinTrackSize do
    begin
      x:=300;
      y:=200;
    end;
    with ptMaxTrackSize do
    begin
      y:=MainForm.ClientHeight-53;
      x:=MainForm.ClientWidth - MainForm.Panel1.width - 4;
    end;
  end;
end;
//=========================================================

procedure TMDIChild.FormCreate(Sender: TObject);
begin
   align:=alClient;
   BorderIcons := BorderIcons - [biMaximize];
   windowState:=wsNormal;
end;