Planning for storage and sizing databases are two things I frequently see companies have trouble with when they plan or roll out their Exchange organizations. Unfortunately, most of us don't have "magic" company crystal balls to peer into and see our organization's requirements for storage, databases, and mailbox sizes.
Mailbox and database sizes can generate some zealous discussions that can actually stall an entire deployment or migration because no one can reach a consensus. Exchange Server administrators frequently want to keep the total size of the databases very small to minimize backup and recovery times. Users want larger allowable mailboxes.
New technologies on the Exchange Server side — such as improved caching, database replication, and Exchange snapshots — may permit larger databases, and technologies such as Unified Messaging (voicemail and faxes in the user's mailbox) and Office Communication Server may require even more storage. Archival technologies, on the other hand, may allow you to reduce the overall storage requirements but still allow users access to historical data.
I am an advocate of giving users the space that they need to do their jobs effectively. If that means giving each user a 5GB mailbox so that every piece of information they need is at their fingertips, indexed, and ready to use via Outlook, Outlook Web Access, or Windows Mobile, then so be it. They should be able to have that storage space if they are ...