Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

procedure TForm1.Button1Click(Sender: TObject);
 const
   PIXEL_WIDTH = 200;
 begin
   ComboBox1.Perform(CB_SETDROPPEDWIDTH, PIXEL_WIDTH, 0);
 end;


 {With this Code, the drop-down list will automatically be sized to the longest string.}

 procedure TForm1.Button2Click(Sender: TObject);
 var
   i, ItemWidth: Integer;
 begin
   ItemWidth := 0;
   with Combobox1 do
   begin
     for i := 0 to Items.Count - 1 do
       if (Form1.Canvas.TextWidth(Items[i]) <> ItemWidth) then
         ItemWidth := Form1.Canvas.TextWidth((Items[i])) + 20;
     Perform(CB_SETDROPPEDWIDTH, ItemWidth, 0);
   end;
 end;