Something like this.
<?php if( have_rows('repeater_field_name') ): ?>
<?php while ( have_rows('repeater_field_name') ) : the_row(); ?>
<?php
$attachment_id = get_sub_field('file_upload');
$url = wp_get_attachment_url( $attachment_id );
$title = get_the_title( $attachment_id );
// part where to get the filesize
$filesize = filesize( get_attached_file( $attachment_id ) );
$filesize = size_format($filesize, 2);
?>
<!-- display a sub field value -->
<div class="item-upload">
<?php the_sub_field('file_upload'); ?> (<?php echo $filesize; ?>)
</div>
<?php endwhile; else : ?>
<?php endif; ?>
