You can configure an Exchange Store size limit for each Exchange database by creating a DWORD registry key named “Database Size Limit in GB”. This key should be created under the following location:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Private-GUID HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Public-GUID
Use a decimal value of 40 for a database limit of 40GB. The default limit is 18GB