function change_wp_search_size($query) {
    if ( $query->is_search ) // Make sure it is a search page
        $query->query_vars['posts_per_page'] = 10; // Change 10 to the number of posts you would like to show. User -1 to show ALL 

    return $query; // Return our modified query variables
}
add_filter('pre_get_posts', 'change_wp_search_size'); // Hook our custom function onto the request filter