For the best calendaring experience, a user's calendar files should not exceed 3 - 4mb. SmarterTools recommends that users enable Calendar Auto-Clean to keep past calendar events from cluttering up their calendar views and exceeding the recommended size limit.
System Administrators can set the default Calendar Auto-Clean setting for a domain within the domain's configuration settings. The User Defaults template can then set a user's Calendar Auto-Clean setting to use that Domain Default or set it to Never, 3 Months, 6 Months or 12 Months. Domain Administrators may also modify this setting per user in the user's configuration options.
Please note that although System and Domain Administrators can enable Calendar Auto-Clean for a user by default, the user may still adjust the setting or turn it off within their Calendar Settings. Therefore, it is important to monitor users' calendar files periodically and contact users that are exceeding the recommended limit.