Could someone explain to me how I can order documents on Firebase by the size of a list?

I have this 'rec' list on several items, but I can only sort it as:

```
body: FutureBuilder<QuerySnapshot>(
              future: Firestore.instance
                  .collection("products")
                  .document(snapshot.documentID)
                  .collection("items")
                  .orderBy("rec")
                  .getDocuments(),

```

<https://i.stack.imgur.com/3yV9D.png>