I adjusted my pool size by modifying the default\conf\standardjboss.xml file
Here is a sample entry for the message beans...
The default setting was in fact 15. I bumped mine up to 50 and when I sent over about 60 messages in a tight loop from the producer, JBoss just locked up on me and started throwing timed out errors...So I'm trying to see what other things I need to adjust. Hope this helps though!
This is the ServerSessionPool.
In 3.2 the config is on the invoker-proxy-binding, see