Add up the sizes of all the tables (including there indexes) for existing data. Then estimate how many rows/size you will need 12-18 months out and make the database file(s) big enough to accommodate that right now. Free space in database files is VERY important to ensure actually-sequential pages AND avoid fragmenting your OS file system as well.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service