To determine the tablespace size on Oracle can be used the following query:

select df.tablespace_name "Tablespace",
   totalusedspace "Used MB",
   (df.totalspace - tu.totalusedspace) "Free MB",
   df.totalspace "Total MB",
   round(100 * ( (df.totalspace - tu.totalusedspace)/ df.totalspace)) "Pct. Free"
from (select tablespace_name,
		   round(sum(bytes) / 1048576) TotalSpace
	  from dba_data_files 
	 group by tablespace_name) df,
   (select round(sum(bytes)/(1024*1024)) totalusedspace,
	  from dba_segments 
	 group by tablespace_name) tu
where df.tablespace_name = tu.tablespace_name 
and df.totalspace <> 0;