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 之後，如果空間不能回收，需要執行西面語句

  清除當前使用者的回收站：purge recyclebin;

  清除所有使用者的回收站：purge dba_recyclebin;