@Override
    public void start() throws IllegalStateException {
        super.start();

        // Add this module as a user event listener so we can delete
        // all offline messages when a user is deleted
        UserEventDispatcher.addListener(this);
        //start timer if enabled
        if (OFFLINE_AUTOCLEAN_ENABLE.getValue())
        {
            setTimer();
        }
    }
Returns the approximate size (in bytes) of the XML messages stored for all
users.

@return the approximate size of all stored messages (in bytes).
xmppserver/src/main/java/org/jivesoftware/openfire/OfflineMessageStore.java