Check number of objects and size of all schema users

set pages 999
col "size MB" format 999,999,999
col "Objects" format 999,999,999


select  obj.owner "Owner"
, obj_cnt "Objects"
, decode(seg_size, NULL, 0, seg_size) "size MB"
from  (select owner, count(*) obj_cnt from dba_objects group by owner) obj
, (select owner, ceil(sum(bytes)/1024/1024) seg_size
  from dba_segments group by owner) seg
where   obj.owner  = seg.owner(+)
order by 3 desc ,2 desc, 1


sample output

Owner Object sizeMB
-------- ------- --------
A          900      700 
sys       29857   22623
B          950       9502