Postgres itself has limits that Neon cannot help: https://www.postgresql.org/docs/current/limits.html. Neon currently serves TB-sized databases. Our storage team has put in a ton of work to enable that. Neon does not currently support something like Citus. Neon has cheap read replicas however, which can allow your primary to just service writes.
I can't find where I read it but my current understanding is that you can't. You can scale to the largest AWS instance available for writes. They allow for automatic read replicas which I imagine can scale horizontally?