I moved this post to a new thread so that the original one can be kept more organized, here is a link back to it:
showthread.php?t=11072
It sounds like you are very close to your solution, they key with this type of setup is making sure that you have another recordset on the page that will be filtered on the id of the size. Once you have the extra recordset like this you can edit your add to cart server behavior and in the bindings area pick the column from the cart and use the lightning bolt to select the corresponding value from the new filtered recordset.
