I have been through a fairly long process already, using the DTK and thought that I was finished, but for a couple of things:

I have cloned the code to read eg:
<legend><?php echo CATEGORY_VAT_NUMBER; ?></legend>
<label class="inputLabel" for="vatnumber"><?php echo ENTRY_VAT_NUMBER; ?></label>
<?php echo zen_draw_input_field('vatnumber', '', zen_set_field_length(TABLE_ADDRESS_BOOK, 'entry_vatnumber', '12') . ' id="vatnumber"') . (zen_not_null(ENTRY_VAT_NUMBER_TEXT) ? '<span class="alert">' . ENTRY_VAT_NUMBER_TEXT . '</span>': ''); ?>
having replaced the original: "entry_company" etc. I also created a field in the database table address_book called "entry_vatnumber" , but can't find the string that defines the box size:
<input type="text" name="company" size = "33" maxlength = "32" id="company" />
I have placed all my mods into:
includes/templates/template_default/templates/parafanaylya/tpl_modules_address_book_details.php
includes/templates/template_default/templates/parafanaylya/tpl_modules_checkout_new_address.php
includes/templates/template_default/templates/parafanaylya/tpl_modules_create_account.php
admin/includes/languages/parafanaylya/english.php
and about 5 others