Heroku docs say "enforced row limits of 10,000 rows for hobby-dev plan" but what does that mean in terms of size? How many Gb is that?
Other providers are clear on their free tier limits: Google Firestore 1Gb, FaunaDB 5Gb, AWS DynamoDB 25Gb.
How much space does Heroku Postgres allow under the hobby-dev plan?
There isn't a limit to disk size of the data stored, only the number of rows for the hobby tier of Heroku Postgres. I have a strong suspicion though that if this is abused you'll probably hear from them, or the database will simply be shut down.