Read this: http://docs.getshopped.org/documentation/creating-variations/
Picture this: James wants to create a store to sell T-shirts. Once he
starts building his product catalog, he realizes that he needs a way
for customers to specify what size shirt they want. He shudders to
think that he might need to create a separate product listing for each
size of each shirt. Not only would that be extremely time-consuming,
it would be confusing for potential customers. Isn’t there an easier
way? Yes! Variations are exactly what James needs. By creating a
variation for size, he can create one listing for each shirt and have
customers simply choose a size from a drop-down list. As an added
benefit, if he needs to charge extra for an irregular size, variations
can handle that, too.
Beyond that,
There is this wp-ecommerce core filter Located in: cart.class.php;
function wpsc_cart_shipping() {
global $wpsc_cart;
return apply_filters( 'wpsc_cart_shipping', wpsc_currency_display( $wpsc_cart->calculate_total_shipping() ) );
}
Which means you could filter the price based on a variable,
function my_variable_price(){
//do your logic here
//based on size selected
}
add_filter('wpsc_cart_shipping', 'my_variable_price');
This is intended as a guide, not a 1:1 solution, since your problem is localized to your use-case, we’d need more information about how you are setting size options. Whether its held in post meta, or via some other means. But, I’d seriously look into the first link provided before going down this path.