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