Increasing the size of the private information store

1. On the computer that is running Exchange Server, open a registry editor such as Regedit.exe or RegEdt32.exe.
2. Navigate to: HKLM\ System\ CurrentControlSet\ Services\ MSExchangeIS\ ServerName\ Private-
3. Right-click MailboxStoreGUID, point to New, and then click DWORD Value.
4. For the new DWORD value, type Database Size Limit in Gb.

Double-click Database Size Limit in Gb. In Value data, type an appropriate value for maximum database size in GB (decimal value - less than 75). Click OK.

Also can be seen here:

http://support.microsoft.com/kb/912375