You need to specify the "size" column as unique in the cart object so that it will know to add a new row if the size has changed in an add to cart.
You can also specify to increase the quantity when submitting the same item from the add to cart page.
I've made those changes. Please download the product_Results.php page and the WA_eCart/eCart1_PHP.php file since I've made those changes for you.