Finding that most content on the web talks about 10G, this is a 9i client so I need a way to size it based on existing rollback. I could just make it as big as all rollback space.
DBMS_UNDO_ADV.RBU_MIGRATION ( 10 G )
Migrating from Rollback Segments to UNDO Tablespace
To move an existing Oracle9i or Oracle Database 10g system from manual undo management (using rollback segments) to automatic undo management requires a shut down and restart—you must shut-down the database, set the undo_management parameter to AUTO, and then restart the database—assuming you've created the UNDO tablespace first, however. To get some guidance about how large to make that UNDO tablespace, you can use a function from the Undo Advisor PL/SQL package created just for that purpose, as in this example:
SQL> set serveroutput on;
SQL> declare utbsize_in_MB number;
3 utbsize_in_MB := DBMS_UNDO_ADV.RBU_MIGRATION;
PL/SQL procedure successfully completed.
In this case, the return value of 187 is the suggested starting size in megabytes for an UNDO tablespace based on current rollback segment utilization in an existing system.