Is there a limit to how big an HTTPSession object can be in WebSphere Application Server

For memory to memory session persistence you can go as big as you want although anything bigger than 10K is problematic.

If the sessions are being persisted to the database, then there is a possible limit.  If you are using single-row, then if the session size is greater than 2MB - we can not store it and therefore if there is a failover, it will not be retrieved.  Using a multi-row schema allows the attributes to be stored in separate rows and so the overall size of the session object can be > 2MB.  The smaller the better as far as replicating sessions.

I would recommend viewing this doc as it has some information about performance for various session sizes for both database and memory to memory replication: ftp://public.dhe.ibm.com/software/webservers/appserv/was/WASV8_SessionReplication_Performance_v1.0.pdf

* Credit to WAS Session subject matter expert Robert Goff