The trick to all of this is having the size select list in the add to cart form on the page. The user will select a size, the values should be set to the id. When the form is posted, meaning the user clicked on the add to cart button, the selected size id will be posted to the page.
You will then need to have another recordset on the page that is filtered on this posted size id. This is the key to getting it to work properly. Once you have this new recordset on the page that is filtering on the id you can then edit your add to cart server behavior. On the bindings tab in the server behavior you can select a cart column then select it corresponding value by using the lightning bolt icon to select the column from your newly filtered recordset.
I think you may be getting close with what you have. If you can post back with the page in question I can take a look to see if I can spot any issues about how it is setup.
