Change Input Field Value using Hook Form Alter (User Login Block)
Put this in your module:

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'user_login_block') {
    $form['name']['#size'] = 10;
    $form['pass']['#size'] = 10;
  }
}


Also hook_form_user_login_block_alter can be used, then you don't need to check the form_id.

Remember to clear the cache, if you are not already using hook_form_alter.