1. 全部表空间的大小
select tablespace_name, sum(bytes)/1024/1024 from dba_data_files group by tablespace_name;

TABLESPACE_NAME      SUM(BYTES)/1024/1024
-------------------- --------------------
UNDOTBS1                   65
SYSAUX                      520
USERS                    6
SYSTEM                      680
EXAMPLE                   100

2. 空闲表空间大小
select tablespace_name, sum(bytes)/1024/1024 from dba_free_space group by tablespace_name;

TABLESPACE_NAME      SUM(BYTES)/1024/1024
-------------------- --------------------
SYSAUX                       34
UNDOTBS1                1
USERS                    1
SYSTEM                    4
EXAMPLE                    22

3. 已使用空间可以这样计算
select a.tablespace_name, total, free, total-free as used from
(select tablespace_name, sum(bytes)/1024/1024 as total from dba_data_files group by tablespace_name) a,
(select tablespace_name, sum(bytes)/1024/1024 as free from dba_free_space group by tablespace_name) b
where a.tablespace_name = b.tablespace_name;

TABLESPACE_NAME       TOTAL       FREE     USED
-------------------- ---------- ---------- ----------
SYSAUX                520    33.6875   486.3125
UNDOTBS1             65      1       64
USERS               6.25       1.25        5
SYSTEM                680        3.5    676.5
EXAMPLE             100    21.5625    78.4375

更具体的sql语句：
select a.tablespace_name, total, free, total-free as used, substr(free/total * 100, 1, 5) as "FREE%", substr((total - free)/total * 100, 1, 5) as "USED%" from 
(select tablespace_name, sum(bytes)/1024/1024 as total from dba_data_files group by tablespace_name) a, 
(select tablespace_name, sum(bytes)/1024/1024 as free from dba_free_space group by tablespace_name) b
where a.tablespace_name = b.tablespace_name
order by a.tablespace_name;