Edit Featured Content Slider

I’m trying to edit the featured content slider on my website:

https://dribblingink.com/

I want it to be more of a square and centered on the page. I tried to edit the featured-content.php, but I couldn’t really find anything in there.

<div id="featured-content" class="flex-container loading <?php brunelleschi_banner_class(); ?>">
	<div class="flexislider">
		<ul class="slides">
		<?php
		$myposts = get_posts( array('meta_key' => 'brunelleschi_featured_post_checkbox', 'meta_value' => 'on', 'post_type' => array('post','page')) ); ?>
			<?php foreach( $myposts as $post ) : setup_postdata($post); ?>
				<li class="slide-cell wraplink clearfix" onclick="window.open('<?php the_permalink(); ?>','_self'); event.stopPropagation();">
					<?php if (has_post_thumbnail( $post->ID ) && ( $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) ){
						$header_image_width = (HEADER_IMAGE_WIDTH < 960) ? 960 : HEADER_IMAGE_WIDTH;
						echo get_the_post_thumbnail( $post->ID, array( $header_image_width, HEADER_IMAGE_HEIGHT ), array( 'class' => 'featured-banner' ) );
					}

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