How to find an Index size

select segment_name,segment_type,bytes/1024/1024 MB
 from dba_segments
 where segment_type='INDEX' and segment_name='your_index_name';

How to find a table size

select segment_name,segment_type,bytes/1024/1024 MB
 from dba_segments
 where segment_type='TABLE' and segment_name='your_table_name';

Script to see Database Size

COLUMN "Total Mb" FORMAT 999,999,999.0
COLUMN "Redo Mb" FORMAT 999,999,999.0
COLUMN "Temp Mb" FORMAT 999,999,999.0
COLUMN "Data Mb" FORMAT 999,999,999.0


SELECT (SELECT Sum(bytes / 1048576)
        FROM   dba_data_files) "Data Mb",
       (SELECT Nvl(Sum(bytes / 1048576), 0)
        FROM   dba_temp_files) "Temp Mb",
       (SELECT Sum(bytes / 1048576) * Max(members)
        FROM   v$log)          "Redo Mb",
       (SELECT Sum(bytes / 1048576)
        FROM   dba_data_files)
       + (SELECT Nvl(Sum(bytes / 1048576), 0)
          FROM   dba_temp_files)
       + (SELECT Sum(bytes / 1048576) * Max(members)
          FROM   v$log)        "Total Mb"
FROM   dual; 

Active session information

    SELECT SID, Serial#, UserName, Status, SchemaName, Logon_Time
    FROM V$Session
    WHERE
    Status='ACTIVE' AND
    UserName IS NOT NULL;