How do i give optimal size to my RB segmennts

[marist89]

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