How to increase the size of the JVM heap in EAP ?

Issue

• Unable to increase the size of the JVM heap.
• While setting JVM's Xmx to 32 GB or more , there's an error appearing in the log :

Starting jboss-as: JVMJ9GC028E Option too large: '-Xmx'
JVMJ9VM015W Initialization error for library j9gc26(2): Failed to initialize
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

• While running java-1.5.0-ibm-1.5.0.11.2-1jpp.1.el5 under Red Hat Enterprise Linux version 5 x86_64 kernel , can not set Xmx to much more than 3750mb. Error is as follows:

JVMJ9GC028E Option too large: '-Xmx4096m'  
JVMJ9VM015W Initialization error for library j9gc23(2): Failed to initialize; unable to parse command line