That all depends on how MyEclipse starts JBoss AS. If you are running JBoss AS manually from a command line (not using MyEclipse or any other IDE), you can edit the jboss-home/run/run.conf (or run.conf.bat) file. I don't use MyEclipse, but if is somewhat close to Eclipse, then in the Servers View, double-clcik the Jboss AS server and then click the Open Launch Config link. The config dilao shows, you can set the heap size on the Arguments tab.