TIMED_STATISTICS won't help here, you're looking at the optimal size of your rollback segments. The optsize field will show up regardless of whether you have TIMED_STATISTICS on or off.
To set the optimal size of a rollback segment, issue:
alter rollback segment r01 storage (optimal 8M);
select n.name, s.rssize/1024/1024 rollback_mb, s.optsize, s.extents, s.xacts current_transactions, s.waits, s.gets, s.status, s.extends no_times_extended, s.shrinks no_times_shrunk
from v$rollname n, v$rollstat s
where n.usn = s.usn
order by n.name
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