Thanks for the link that helps. It looks like you have got the size list updating the price on the product page. Is this working correctly for you, are you getting the right options in the size list and the correct price when you select the size?
If so then I think you are mostly there. It looks like you are doing some custom stuff when the item is added to include the additional charge or difference in charge for the larger size. Instead of doing it this way I think you can make it add the selected size/price directly without the extra size items in the cart.
To do this you would want to alter the add to cart form for the products on the page. You would want to add in a new hidden form element. You can then use the js for displaying the price to set the value of this hidden form element. Then you can update your add to cart server behavior to get the price from this element.
Depending on how you have the prices and sizes organized in your db you could set the select list to have a value of the id for that size/price. Then add in another recordset to the page that would select from this table and filter on the posted id from the select list. This way you can use the recordset values for the price and size bindings in your cart. Please post back if you have any further questions on either of these approaches.