function mysite_form_alter(&$form, &$form_state, $form_id) 
{
  switch ($form_id) 
  {
    case 'user_profile_form':
      $form['account']['name']['#size'] = 30;
      $form['account']['mail']['#size'] = 30;
      break;
  }
}

http://geeksandgod.com/forums/dynamic-website-forums/all-about-drupal/resize-registration-email-and-name-textfield-drupal-6

Resizing fields using a custom module

You could also resize the field using a custom module.
For example, if you created mysite.module, you could use something like this to reduce the field size to 30 characters

Drupal example on how to use form alter hook

form, drupal

PHP