After years of consideration, bloat is now controllable by altering the size of the undo tablespace.
Hm. Well, fwiw the situation is rather more complicated than that. You're correct that you can create a fixed size undo tablespace in which case Oracle will tell you approximately how much retention period you can rely on given that size. But alternately you can set the undo tablespace to be autoextend and Oracle will tune it to ensure there's enough retention to cover the longest-running query in the system. You can also set a time-based parameter (undo_retention) to ensure there's always at least enough data retained to cover a fixed amount of time for flashback (Oracle's equivalent to our time travel feature in ancient versions of Postgres).
