you would need to have eCart to implement this.
In that case the cart object would need to have a height width and length column, the products table would also need to have these columns and the add to cart buttons would need to be modified so that the values in the database are added to the cart when the item is added. Then the height weight and length for UPS could be bound to the cart columns that hold those values.
You would also need to modify the Power Store admin pages using DataAssist so you could add the length width and height values when inserting a product or updating it.