Definido o tamanho mínimo e máximo de um formulário

Unit Unit1;

interface

uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
TForm1 = class(TForm)
private
{ Private declarations }
procedure WMGetMinMaxInfo(var MSG: TMessage); message WM_GetMinMaxInfo;
public
{ Public declarations }
end;

var
Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.WMGetMinMaxInfo(var MSG: TMessage);
begin
inherited;
with PMinMaxInfo(MSG.lparam)^ do begin
ptMinTRackSize.X := 300;
ptMinTRackSize.Y := 150;
ptMaxTRackSize.X := 350;
ptMaxTRackSize.Y := 250;
end;
end;
end. //ou, altere a propriedade constraints( que exite apenas a partir da versão 4.0.