As of JDK6U18 following are configurations for the Heap Size.
In the Client JVM, the default Java heap configuration has been modified to improve the performance of today's rich client applications. Initial and maximum heap sizes are larger and settings related to generational garbage collection are better tuned.
The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte. For example, if your machine has 128 megabytes of physical memory, then the maximum heap size is 64 megabytes, and greater than or equal to 1 gigabyte of physical memory results in a maximum heap size of 256 megabytes. The maximum heap size is not actually used by the JVM unless your program creates enough objects to require it. A much smaller amount, termed the initial heap size, is allocated during JVM initialization. This amount is at least 8 megabytes and otherwise 1/64 of physical memory up to a physical memory size of 1 gigabyte.
Source : http://www.oracle.com/technetwork/java/javase/6u18-142093.html