You need to add the <SIZE /> tag into your Cluster configuration file.
Cluster configuration file varies according to JBoss release: in this article you can check which is the correct Cluster configuration file:
Now look for the "JGroups protocol configuration.". In recent jBoss release the Attribute which holds the configuration is "PartitionConfig".
Add the <SIZE /> empty tag in the Config section.
<server> <attribute name="PartitionConfig"> <Config> ........ <SIZE /> </Config> </attribute >
this will dump the Http session size in the following format:
12:36:19,605 TRACE [SIZE] size of message is 2850, 3 headers