What is the maxium heap size we can allocate to single JVM?

in one line......

it depends on your hardware configuration 

If you're running 32-bit WAS on UNIX, it's 2GB. If running 64-bit WAS it's effectively unlimited, limited to the amount of physical memory installed. If on UNIX, you'll need to subtract several GBs for operatnig system, etc.

I f you are ruunig on WAS 32 bit then the theoritically maximum heap size you can use would be 4 GB ( 2 power 32) and if you are running on WAS 64 bit then its in teraa bytes (2 power 64).

 

However you should be aware of the performance impact of garbage collection when allocating large amounts of RAM to your java process.
The more RAM you allocate, the longer the garbage collection pauses get (stop the world). On our HP-UX IA64 Systems, we tend not to use more than 2GB HEAP (which results in about 4GB total RSS process memory) for our web application servers, as FULL GC pauses get as high as 6-8 seconds (which is already bad from a user perspective, as the applications become unresponsive for this time).

I sugest you to follow this document to determine the heap usage in your application serving environment. Once the mertrics and trends are collected and analyzed.. it will be easy to decide what would be the good heap settings. 

www.websphereusergroup.org/josephamrithr... 

This might help too:

publib.boulder.ibm.com/infocenter/javasd...
 

Perhaps this will help, too:
www-304.ibm.com/support/docview.wss?uid=...

Thanks to All for the reply.