This is an Android example app for how to use the FirestoreDocument-Android library. This library answers the question "How to get the size of a Cloud Firestore document"?
In order to make it work, you need to create a Firebase project and a very simple Cloud Firestore database with only a single document that looks like this:
When opening the app, the result that you'll see is the size of my_task_id document that is set to a TextView:
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Note: All documents size returned by this library are in bytes, the example above shows 147 bytes