Is there any way to adjust the items per block options?
Thanks to the answer @Berdir provided, I've been able to replace the restrictive select field with a more flexible number field.
In mymodule.module
The form_id is block_form (andpanels_edit_block_form and panels_add_block_form for Panelizer). Change the #type to number to make it so that you can input a number. Unset the #options to avoid validation errors (Invalid choice).
/**
 * Implements hook_form_alter.
 *
 * Changes the input type of the &quot;Items per block&quot; setting of
 * a view-block in Blocks/Panelizer from 'select' to 'number',
 * so that amounts other than 5, 10, 20 and 40 can be chosen.
 */
function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (in_array($form_id, ['block_form', 'panels_edit_block_form', 'panels_add_block_form'])) {
    if (isset($form['settings']['override']['items_per_page'])) {
      $form['settings']['override']['items_per_page']['#type'] = 'number';
      unset($form['settings']['override']['items_per_page']['#options']);
    }
  }
}