You are on the rite track here, but rather than have the recordset reference in the calculation you would want to have another column in your cart to store this value, then when you update the size you will also need to update this other column.
Getting the other column to update to the proper value when you update the selected option would require some js coding. You could have a hidden form element for the other column and a display of the value so that you can see it, then when you update the selected option you could update the value of the hidden form element for the other column. Then when you click on the update button for the cart the columns should all be updated to the new values and the calculations should show you the correct price based on the selected option.