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;

Drop Table 之后，如果空间不能回收，需要执行西面语句

清除当前用户的回收站：purge recyclebin;

清除所有用户的回收站：purge dba_recyclebin;