Hi

We can change jvm heap size with following runtime parameter :

java -Xms -Xmx

example :

java -Xms32m -Xmx512m

you can see these links for more parameter(switch) :

http://performance.netbeans.org/howto/jvmswitches/index.html
http://java.sun.com/javase/technologies/hotspot/vmoptions.jsp
http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html