Edit the following entry in the run.conf file. But if you have multiple JVMs running on the same JBoss, you might want to run it via command line argument of -Xms2g -Xmx4g (or whatever your preferred start/max memory settings are.
if [ "x$JAVA_OPTS" = "x" ]; then
JAVA_OPTS="-Xms2g -Xmx4g -XX:MaxPermSize=256m -Dorg.jboss.resolver.warning=true