Locate the registry key for the mail store by browsing to the below location.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<SERVER NAME>\Private-00000-0000-000e-0000-0000000000 (To locate the private guid for your mailstore click here)
If the value ‘Database Size Limit in GB’ already exists, change the value to the new size in GB.
If the value does not exist within the registry, add the DWORD value ‘Database Size Limit in GB’ and set the value as required. Select decimal before setting the value.
For the change to affect straight away, you must restart the information store service. An event id of 1216 will be logged within the event log to confirm the change has been made.
If you can not restart the information store service at this time, you can wait until the automated Exchange online defrag takes place over night.