I dont understand your question, are you asking how to size or you mean why you hit the error....?
Now I know why I hit the error... Thanks for the help.
The question was about basics in sizing Rollback segments and things to keep in mind.
You can if you think you can.
Sizing RBS is an iterative process.
Query from v$rollstat when the db has high volume of transactions.
The aveactive column value will be the starting size (for initial and next) for the RBS.