For a reference selector the advise is to use a page for selecting options if there are more than 20 options.
There is a limit for how many options will be shown. The same holds true for a filter of a datagrid and there is no setting that will influence this behavior. To make sure the user can filter on all the options change the search field to a text box. This will also help improve the performance of the page with the datagrid as the options no longer have to be generated and rendered.