I doubt the size of mail Stores will change much. If anything, it may grow
larger if you're using Unified Messaging. You will be able to have more
number of smaller Stores and as many as 50 Storage Groups.
Store Management in terms of ability to granularly manage folders in user
mailboxes, organizational folders, et al are indeed better.
Disk IOPS requirements should reduce significantly - you will be able to
host more users on a set of spindles than you did before.

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Bharat Suneja
MVP - Exchange
www.zenprise.com
NEW blog location:
www.exchangepedia.com/blog
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> Does anyone happen to know if the mail stores in Exchange 2007 compared to
> 2003 will be comparably smaller, or any way to calculate what a current
> 2003
> mail store might be once everything is transitioned over to 2007? I had
> read
> that store management would be far better in 2007.