Use following query to check table size in your schema by changing schema name in WHERE Clause.
select segment_name,segment_type,bytes/1024/1024/1024 GB,ds.*
from dba_segments ds
where segment_type='TABLE' and owner='SCOTT' order by 3 desc
Replace "SCOTT" with your schema/user name to get objects from your schema.
select segment_name,segment_type,bytes/1024/1024/1024 GB,ds.*
from dba_segments ds
where segment_type='TABLE' and owner='SCOTT' order by 3 desc
Replace "SCOTT" with your schema/user name to get objects from your schema.