TWin32ComboBoxStringList = class(TWin32ListStringList)
  private
    FDropDownCount: Integer;
    procedure SetDropDownCount(const AValue: integer);
  protected
    function GetComboHeight: integer;
    ...
    procedure UpdateComboHeight;
  public
    ...
    property ComboHeight: integer read GetComboHeight;
    property DropDownCount: integer read FDropDownCount write SetDropDownCount;
  end;

procedure TWin32ComboBoxStringList.SetDropDownCount(const AValue: integer);
begin
  FDropDownCount := AValue;
  UpdateComboHeight;
end;

function TWin32ComboBoxStringList.GetComboHeight: integer;
var
  R: TRect;
begin
  if TCustomComboBox(FSender).Style = csSimple then
  begin
    // combobox workaround:
    // if style = csSimple follow the LCL height.
    Result := FSender.Height;
  end else
  begin
    Windows.GetClientRect(FWin32List, @R);
    if Count = 0 then
      Result := R.Bottom + Windows.SendMessage(FWin32List, CB_GETITEMHEIGHT, 0, 0) + 2
    else
      Result := R.Bottom + FDropDownCount * Windows.SendMessage(FWin32List, CB_GETITEMHEIGHT, 0, 0) + 2;
  end;
end;

procedure TWin32ComboBoxStringList.UpdateComboHeight;
var
  Width, Height: integer;
begin
  Width := FSender.Width;
  Height := ComboHeight;
  SetWindowPos(FSender.Handle, 0, 0, 0, Width, Height, SWP_NOZORDER or SWP_NOACTIVATE or SWP_NOMOVE);
end;