"too big" transactions

How does PostgreSQL handles a "too big" transaction?

By that I mean a transaction which, after a certain point, there will be no
way to roll back. On PgSQL, maybe that only happens when the disk fills. Is
there a configurable "size" limit for a single transaction?

In addition, what happens if the disk fills up? Postgres is able to roll
back, right?

I'm assuming you can prevent the disk from actually filling up (and crashing
the whole server) by turning on quotas for the postgres super user, so that
only pgsql would complain. Please correct me if I'm wrong.