How to set different featured image for a particular category in grid format

I have created a new category-artists.php based on the archive.php so I can set a different featured image size for this category only. But I am not able to set the size I don’t see where the featured image is being called in this part of the code

 global $posts;
                        $post_ids = array();
                        foreach($posts as $post) $post_ids[] = $post->ID;
                        if(!empty($post_ids))
                        {
                            $atts   = array(
                                'type' => 'grid',
                                'items' => get_option('posts_per_page'),
                                'columns' => 5,
                                'class' => 'avia-builder-el-no-sibling',
                                'paginate' => 'yes',
                                'use_main_query_pagination' => 'yes',
                                'custom_query' => array( 'post__in'=>$post_ids, 'post_type'=>get_post_types() )
                            );
                            $blog = new avia_post_slider($atts);
                            $blog->query_entries();
                            echo "<div class='entry-content-wrapper'>".$blog->html()."</div>";

I see this $blog – new avia_post_slider($atts) where is this being called for. I need set a different featured image size other than the size-portfolio_small its currently set.

Thanks and appreciate it