I don't have an out-of-the-box solution for you but:
1) if you are using JBoss 3.2.3 (or higher) the web-console can give you a couple of interesting numbers (one of which is total memory used)
2) for JBoss versions superior to 3.2.2 you can uncomment a BeanCacheMonitor service in the standardjboss.xml. This MBean then becomes available in the JMX console and shows you how many beans are cached. This should allow you to roughly estimate the memory size needed (if you know how big your beans are, that is)
Hope this helps,
Thanks, my client is running 3.2.1, but I'll have a look at 3.2.3.
Seems un-intuitive not to have this info.