How do I know the Http Session replication size?

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.

 <attribute name="PartitionConfig">
           <SIZE />

 </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