you need to edit an override copy of includes/modules/product_listing.php
at about lines 98 and 100 you will see two long lines of code beginning with $lc_text = '<h3
Near the end of each of those lines you will see this segment of code:
PHP Code:
zen_trunc_string(zen_clean_html(stripslashes(zen_get_products_description($listing->fields['products_id'], $_SESSION['languages_id']))), PRODUCT_LIST_DESCRIPTION)
which displays a part of the product description, stripped of paragraph and break tags.
To make if display the entire product description as you entered it for the product, replace that segment with
PHP Code:
zen_get_products_description($listing->fields['products_id'], $_SESSION['languages_id'])