Caption on CKEditor with Media embed only allows 128 characters
This should do the trick no need to install any module, substituting 500 with whatever you want the limit to be

 function mymodule_form_entity_embed_dialog_alter(&amp;$form, $form_state, $form_id) {
      if(isset($form['attributes']['data-caption'])) {
        $form['attributes']['data-caption']['#maxlength'] = '500';
      }
    }