This was an unanswered question on the WP e-Commerce blog and we needed to create a solution to it for a client project. Oddly WP e-commerce doesn't limit the number of characters shown in 'wpsc_the_product_description();' out of the box and the result is that when you show product descriptions they can look untidy in length and layout. Our solution is to apply a custom filter to trim/limit the words shown and then show a 'read more' link for the post/custom post.
Custom Excerpt Length
Alternatively for a custom excerpt length you can use:-
Change the Excerpt Length in your Template
You can also use the following php snippet to change the excerpt length directly in your template without adding a function to functions.php:-
The same snippet can be amended to use the 'get_the_content' function to trim the content instead.