Understanding Mailbox Database and Log Capacity Factors for Exchange Storage Sizing

The below link explains the factors that you should consider when planning mailbox database and log capacity as part of mailbox server storage design in Microsoft Exchange Server 2010.

•   Mailbox storage quotas
•   Database white space
•   Mailbox database recoverable items
•   Actual mailbox size
•   Content indexing
•   Offline database maintenance
•   Recovery database
•   Database size
•   Database growth overhead

http://technet.microsoft.com/en-us/library/ee832796.aspx.