You’ve probably read by now that Exchange 2003 Service Pack
2 increases the message store size limit to 75GB on the Standard Edition.  This is a good thing for many smaller IT
shops that do not need the other functionality provided by Exchange Enterprise
but that are running out of available space in the information store.

After you install Service Pack 2, you might expect an almost
fivefold increase right away, but if you look closely, you’ll see that the size
of your information store is only 18GB — far from the promised 75GB! What
gives?

Believe it or not, this is a good thing.  In Service Pack 2, the Exchange developers
have provided you with the ability to size the information store to any size
you like between 1 and 75 GB, and they chose 18GB as the default (16GB + about
10% for a warning buffer).  The reason:
Not everyone has disks on their Exchange server that could accommodate a 75GB
information store.  For these users,
increasing the store limit all the way to the maximum allowed could result in
the Exchange server exhausting its storage resources before getting anywhere
close to a maximum.

To change the size limit:

  1. Start
    the registry editor.
  2. Browse
    to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\[ServerName]\[GUID of
    private store] or
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\[Servername]\<GUID of public store>3Choose Edit
    > New > DWORD Value.
  3. Name
    the new value “Database Size Limit in GB”
  4. Edit
    the new value and give it a size between 1 and 75 GB.
  5. Restart
    the Information Store service.

This key will also work on the Enterprise Edition of
Exchange if you want to limit the size of a database to less than 8TB. If you
ever have to recover Exchange with the /disasterrecovery
switch, you will need to manually recreate these keys.