// .h
// 一個與 TCustomComboBox 相同的物件，只是可以取出 ListHandle
class TMyComboBox : public TCustomComboBox
{
public:
  HWND __fastcall GetListHandle(void)
  {
    return this->ListHandle;
  }
public:
        inline __fastcall virtual TMyComboBox(Classes::TComponent* AOwner)
  : TCustomComboBox(AOwner) { }
  inline __fastcall virtual ~TMyComboBox(void) { }
public:
        inline __fastcall TMyComboBox(HWND ParentWindow)
  : TCustomComboBox(ParentWindow) { }
};

// .cpp
void __fastcall TFormTest::ComboBox1DropDown(TObject *Sender)
{
  TMyComboBox *MyComboBox=(TMyComboBox *)ComboBox1;
  HWND hwnd=MyComboBox->GetListHandle();
  if(hwnd!=NULL)
  {
    SetWindowPos(hwnd, NULL, 0,0,
      300, // 新的寬
      400, // 新的高
      SWP_NOMOVE | SWP_NOZORDER);
  }
}