Sizing the storage for an Exchange 2007 mailbox server can be tricky. There are lots of aspects you must take into account, such as mailbox size, deleted item retention period, user profile, backup policy and so on...
Fortunately, Microsoft has this Excel spreadsheet, Exchange 2007 Mailbox Server Role Storage Requirements Calculator, that can perform all the calculations for you.
This helpful tool has been recently updated and just got better. For a list of the new improvements, please read 7/5/2007 update to the Mailbox Server Role Storage Requirements Calculator.
For the explanatory article of the calculator, go to Exchange 2007 Mailbox Server Role Storage Requirements Calculator.