select aa.owner, aa.table_name, aa.num_rows as "Número de linha", round((bb.bytes / (1024*1024))) as MB
from    (select a.owner, a.table_name, a.num_rows from dba_tables a) aa,
        (select b.owner, b.segment_name,  b.bytes from dba_segments b where b.segment_type = 'TABLE') bb
where aa.owner =  bb.owner
and   aa.table_name = bb.segment_name
and   aa.owner like 'DB%'
and   aa.num_rows is not null
order by MB desc