//Tamanho do banco
select sum(bytes) / 1024 / 1024 / 1024 tamanho_GB from dba_segments;

//TAMANHO DO SCHEMA 
SELECT
SEGMENT_TYPE, SUM(BYTES) SIZE_IN_BYTES
FROM DBA_SEGMENTS
WHERE OWNER = 'NOME_DA_SCHEMA'
GROUP BY
SEGMENT_TYPE

//TAMANHO DA TABELA 
SELECT * FROM 
(SELECT OWNER, SEGMENT_NAME, TRUNC(SUM(BYTES)/1024/1024/1024,2) "SIZE GB"
      FROM DBA_SEGMENTS
      WHERE SEGMENT_TYPE = 'TABLE' AND OWNER = 'XXX'
      GROUP BY SEGMENT_NAME, OWNER
      ORDER BY 3 DESC)
      WHERE ROWNUM <= 100;
	  
SELECT * FROM 
(SELECT OWNER, SEGMENT_NAME, TRUNC(SUM(BYTES)/1024/1024/1024,2) "SIZE GB"
      FROM DBA_SEGMENTS
      WHERE SEGMENT_TYPE = 'TABLE' AND 
            OWNER = 'SYS' AND
            SEGMENT_NAME IN ('AUD_POLICY$','AUD_OBJECT_OPT$','AUDIT_NG$','AUDIT_ACTIONS','AUDIT$','AUD_CONTEXT$','AUD$')
      GROUP BY SEGMENT_NAME, OWNER
      ORDER BY 2 DESC)
      WHERE ROWNUM <= 10;