My client has many similar products in one category. How do I lengthen the page or add a page to a category, please?

Unfortunately that's not possible. Not sure if the category setups will change in the future or not, but unless/until they do you are only able to add products to a category or a subcategory. There is no way to alter how long it goes nor can you alter the menu itself for the categories. I also moved your post to the Shopping Cart Creator Pro forum as this question pertains to that program rather than the Designer program