probably doesn't help you having several people responding to the thread at the same time, but in reply to your post # 11.
You are adding add to cart behavior on both the products and product details page
If you have multiple products per page, each with an add to cart button, and each product has different size options, I think that what you have to do is have the product sizes as a nested recordset within the products loop.
( I am assuming that not all products are available in all sizes, eg the sports holdall)