Each instance of the server has a run.bat, which you can edit to specify the heap size. Something like:
rem Sun JVM memory allocation pool parameters. Modify as appropriate.
set JAVA_OPTS=%JAVA_OPTS% -Xms128m -Xmx512m
we have only one run.bat.
But under <JBOSS_HOME>/server/ we have some instances.
Any other ideas?
You can copy the run.bat to run-instanceN.bat etc.