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_gigfrom    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_typefrom    dba_segmentswhere    owner=‘SCOTT‘ group by    segment_type;

Oracle DB , 计算各个用户/schema 的磁盘占用空间