You should trap WM_GETMINMAXINFO message. In your form's class declaration put this:
procedure WMGetMinMaxInfo(var Msg: TWMGetMinMaxInfo); message WM_GETMINMAXINFO; and in the implementation section: procedure TYourForm.WMGetMinMaxInfo(var Msg: TWMGetMinMaxInfo); begin inherited; with Msg.MinMaxInfo^ do begin ptMinTrackSize.Y:=200; ptMaxTrackSize.Y:=400; end; end;