http://www.dba-oracle.com/t_find_size_schema.htm

Question: How do I find the size of a schema in my database?

Answer: It depends on how you define "size". If you want the total disk space consumed by the schema (including indexes, tablespace free space), then the SQL query is quite simple:

select
 
 
   owner,
   sum(bytes)/1024/1024/1024 schema_size_gig
from 
   dba_segments 
group by 
   owner;

If you just want the size of the schema in terms of the space consumed by table rows, you would use a different query to calculate the schema size:

select 
   sum(bytes)/1024/1024/1024 as size_in_gig, 
   segment_type
 
from 
   dba_segments
where 
   owner=‘SCOTT‘ 
group by 
   segment_type;

Oracle DB , 計算各個用戶/schema 的磁盤占用空間