Display all comments of node without pager (300+ comments)
It's stored in a variable per content type, you can override it like so:

variable_set('comment_default_per_page_CONTENT_TYPE', 500);


Where CONTENT_TYPE is the machine name of the content type.

If you want to change the options in the UI, you can do it through a form alter:

function MYMODULE_form_node_type_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form['#node_type'] == 'foo') {
    $new_options = drupal_map_assoc(array(10, 30, 50, 70, 90, 150, 200, 250, 300, 500, 1000));
    $form['comment']['comment_default_per_page']['#options'] = $new_options;
  }
}