diogobento on "List posts by year, 6 per page."

I've managed to get and list my posts by year, but

Code for listing posts by year:

functions.php

function posts_by_year() {
  // array to use for results
  $years = array();

  // get posts from WP
  $posts = get_posts(array(
    'numberposts' => -1,
    'orderby' => 'post_date',
    'order' => 'ASC',
    'post_type' => 'post',
    'post_status' => 'publish'
  ));

  // loop through posts, populating $years arrays
  foreach($posts as $post) {
    $years[date('Y', strtotime($post->post_date))][] = $post;
  }

  // reverse sort by year
  krsort($years);

  return $years;
}

template file

<?php foreach(posts_by_year() as $year => $posts) : ?>
  <h2><?php echo $year; ?></h2>

  <ul>
    <?php foreach($posts as $post) : setup_postdata($post); ?>
      <li>
        <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
      </li>
    <?php endforeach; ?>
  </ul>
<?php endforeach; ?>

What should I do next so that only 6 posts per page are shown?

Anyone could give me a hint here?
Thanks.