You may need to add in a new column to your order details table to hold the size value. You would then need to modify the store order details server behavior using eCart, IN this server behavior you would just associate the size column from the cart with your new column in your order details table.
Once you do this you will be recording the size along with the other order information. You can add in bindings for the new column to the admin backend or the order history to get back the size.