i think your after this.
in skin1/modules/products options/customer_options.tpl
change
Code:
<input type=text name="product_options[{$product_options[product_option].optclass}]" size=32>
to
Code:
<textarea name="product_options[{$product_options[product_option].optclass}]" cols="32" rows="5"></textarea>
see if that helps