What is the largest possible heap size with a 64-bit JVM?

Question

The theoretical maximum heap value that can be set with -Xmx in a 32-bit system is of course 2^32 bytes, but typically (see: Understanding max JVM

Answer 1

The answer clearly depends on the JVM implementation. Azul claim that their JVM

can scale ... to more than a 1/2 Terabyte of memory

By "can scale" they appear to mean "runs wells", as opposed to "runs at all".