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

procedure TForm1.WMGetMinMaxInfo(var MSG: Tmessage);
Begin
  inherited;
  with PMinMaxInfo(MSG.lparam)^ do
  begin
    with ptMinTrackSize do
    begin
      X := 300;
      Y := 150;
    end;
    with ptMaxTrackSize do
    begin
      X := 350;
      Y := 250;
    end;
  end;
end;