Thanks a lot, tcindie@drupal.org
It works now after follow your suggestion above!
btw. I have another custom field (namely name-text which I like to limit the length to 20 chars), it is in the add-to-cart form at product page, how do I find out the form id and input text field? Where to put the <? //php print_r($node); ?> or
<?php
print($form_id);
?>in order to find the id and field.
Thanks for your help.
Edit:
I found the answer from this mini lesson on hook_form_alter, see http://groups.drupal.org/node/4308
Thanks