this article picked from：http://viralpatel.net/blogs/setting-tomcat-heap-size-jvm-heap-eclipse/
Recently while running Tomcat under Eclipse for one of the web application I was getting Java Heap memory related error java.lang.OutOfMemoryError.
What needs to be done here basically is to increase the jvm heap size. So for increasing the JVM Heap Size of Tomcat in Eclipse we have to set few VM arguments of the tomcat.
Follow the simple steps to change the Heap Size of Tomcat under Eclipse.
1. Open the Server tab in Eclipse and double click the Tomcat server to open Server Configuration.
2. In Server Configuration, click on the Launch Configuration link under General Information.
3. Under Arguments tab, add following values in VM arguments.
-Xms64m -Xmx256m
To know more about -Xm options read this article.