Oracle: Get approximate size of storage used by indexes by a user

SELECT
  SEGMENT_NAME,
  ROUND(BYTES / 1048576)       AS MEGABYTES,
  ROUND(BYTES / 1073741824, 1) AS GIGABYTES
FROM DBA_SEGMENTS
WHERE SEGMENT_TYPE = 'INDEX' AND OWNER = '_YOURUSER_COMES_HERE_'
ORDER BY 2 DESC, 1 ASC;