Hie,
I've a script which give you the growth of your DB during months.

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set pagesize 50000
tti "Database growth per month for last year"

select to_char(creation_time, 'RRRR Month') "Month",
sum(bytes)/1024/1024 "Growth in Meg"
from sys.v_$datafile
where creation_time > SYSDATE-365
group by to_char(creation_time, 'RRRR Month')
/

tti off

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Have fun !!!