More Dynamic Form Response

So currently, I have a food ordering application in Rasa. Everything is going fine, but I have the following issue. All “sides” in the menu must have a size i.e. small, medium, or large fries. Currently, I have 2 list slots: 1 that contains the whole order and 1 list that contains all unresolved sides in it. I want my agent to always ask for a size whenever the list of unresolved sides is empty i.e. in the following interaction:

  1. Bot: What would you like to order? 2: User: I want 1 cheeseburger and 2 fries [set order to [cheeseburger] and unresolved_items to [2 fries]]
  2. Bot: What size would you like your fries? 4a User: Large {Sets the unresolved_item to [], sets order to [cheeseburger, 2 large fries]] 4b User: 1 Large and 1 Medium [Sets the unresolved_item to [], sets order to [cheeseburger, 1 large fries, 1 medium fries]

I want to be able to support bot lines 4a and 4b in the form, so that the user can respond faster. Note that this will eventually become an audio system, so I want to minimize the amount of interactions between the user and the machine. How can I program a form to both accept line 4a and line 4b? I currently have a code that manually extract this from the text i.e. transform “large” to [large] and “1 large and 1 medium” into [1 large,1 medium], but I am not sure what is the best way to use this function in the code. Should I just make this a custom action rather than a form?