To make a change in the size (or any variation) of an Add-On, you will need to edit the Add On variation response via the Customer's account. 

• Go to Customers > Account Management > Search to find the customer and then click on their name to go to their Account Overview page.
• Click on the spy glass icon for the order.
• Click on the pencil icon for the Buy Item you need to change
• Make the necessary updated to the Variation responses and click Save.
• The Enrollment Add On will be updated and show a green (Updated) message to indicate the edit.