Filter: ep_facet_meta_value_max_strlen

Max length of each value in the facet.

To set it to only display 3 characters of each value when the meta_key is my_key:

add_filter(
    'ep_facet_meta_value_max_strlen',
    function( $length, $meta_key ) {
        if ( 'my_key' !== $meta_key ) {
            return $length;
        }
        return 3;
    },
    10,
    3
);

Please note that this value is cached. After adding that code to your codebase you will need to clear WordPress's cache or save a post.