I need to report the database sizing to our management.

Following are the questions:

1) Total space allocated and used for each Database instance (We have 14 each for 3 applications)
2) How is the growth pattern over the last few years. Year to Year change.

My answer is
1) Run the following query in each instance

SELECT 'ALLOCATED ', SUM(bytes) / 1024 / 1024 / 1024 AS "DB_SIZE-GB" FROM dba_data_files
UNION
SELECT 'USED ', SUM(bytes)/1024/1024/1024 from dba_segments;

2) Store this info in a user defined Table and insert a row with the date once a month, starting today.

So I can provide some growth pattern in a year or couple of years from now.

Can you help improve this in any way, since it is going to be sent to the Management.

select sn.begin_interval_time, s.tsname, s.snap_id, s.contents, u.tablespace_size, u.tablespace_usedsize
from DBA_HIST_TBSPC_SPACE_USAGE u,
      dba_hist_tablespace_stat s,
      dba_hist_snapshot sn
where s.snap_id = u.snap_id
and s.dbid = u.dbid
and s.ts# = u.tablespace_id
and sn.snap_id = u.snap_id
and sn.dbid = u.dbid;