Explanation:
It's recommended to use a table space for RMAN with 10M, this table space will be relatively small.
Incorrect Answers:
2: It is more then enough for RMAN table space if you create a table space with 20M size.
3: Size 100M is extremely high level for RMAN table space.
4: Size of table space for RMAN catalog is not related with size of the target database.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 618
Chapter 13: Using Recovery Manager for Backups

---