Hi,
You have to analyse and foresee the growth of the database for the next 5 years (atleast) and based on that you have to keep hard disk space available for database. When you configure database growth option its always recommended that you set it for fix mb refer http://www.extremeexperts.com/SQL/Articles/AutoGrowth.aspx
Coming to the growth of the T-Log file, here is a link you can refer http://technet.microsoft.com/en-us/library/ms365418.aspx and http://hemantgirisgoswami.blogspot.com/2006/03/cause-for-t-log-become-full-and-how-to.html
hth
Hemantgiri S. Goswami | MS SQL Server MVP
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"Humans don't have Caliber to PASS TIME , Time it self Pass or Fail Humans" - by Hemantgiri S. Goswami
http://hemantgirisgoswami.blogspot.com
http://forums.surat-user-group.org/

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