How can I Estimate Table Sizes within Schema (Oracle)
Look at the "dba_segments" view (or user_segments if you don't have dba rights). The following query should give you what you're looking for:

select
  owner as "Schema"
  , segment_name as "Object Name"
  , segment_type as "Object Type"
  , round(bytes/1024/1024,2) as "Object Size (Mb)"
  , tablespace_name as "Tablespace"
from dba_segments
order by owner;