Read this: http://msdn.microsoft.com/en-us/library/ms187445.aspx

However the big factor is how much growth you are going to have from new data. If you have no idea what load you'll have, however people will enter data (or systems will), then there's nothing you can do.

What you can, and should, do is track this and regularly adjust the size of the database. Autogrowth is an emergency option, not the way to manage space.