SQLPlus access to the machine with the database installed. Once logged in, type this query:

    select sum(bytes/(1024*1024)) "Reserved_Space(MB)" from dba_data_files;

Example of output:
Reserved_Space (MB)
------------------
           3916.125

    select sum(bytes/(1024*1024)) "Free_Space(MB)" from dba_free_space;

Example of output:
FREE_SPACE (MB)
--------------
       1807.625