I'd keep your MAXEXTENTS to a nominal size depending on the size of your extents, tablespace size and transaction size. Set your MAXEXTENTS from 200 - 2000 if you'd like. In your case it depends on how large you transaction are likely to grow.

The only thing I'd advise you on is to HAVE a MAXEXTENT set, in order to STOP a rouge transactions from occupying the entire RBS TABLESPACE.

Also Setting your OPTIMAL to small (which I'm not saying you have done, but again depends on you average transaction size) can cause "Snapshot too old", because a shrink can occur on your rollback segment and cause another process using the segment to lose it's consistent view. Very frustrating. In my opinion, the ROLLBACK SHOULD NOT shrirk until an active transaction using it for read consistency is completed. (But I'm not developing the oracle engine). Unfortunalty, Oracle9i didn't fix this issue either, istead of setting an OPTIMAL size, you set a RETENTION_TIME. This will cause the same problem.

Sorry I got a bit off track...