uses
LCLIntf, LCLType;
procedure TForm1.FormCreate(Sender: TObject);
var
i, w, max: Integer;
begin
max := CheckComboBox1.Width;
for i := 0 to CheckComboBox1.Items.Count - 1 do
begin
w := CheckComboBox1.Canvas.TextWidth(CheckComboBox1.Items[i]);
if w > max then
max := w + 10 + GetSystemMetrics(SM_CXMENUCHECK) {check width};
end;
CheckComboBox1.ItemWidth := max;
end;