I have apache-tomcat as my web server. I want to check what heap size is allocated for jvm by linux. Also from where, I can modify it.
You can easily check the heap size memory allocation using JConsole, if you have a path to your jre/jdk set up correctly on the system you should be able to start it with command jconsole
from anywhere.
For managing your heap memory allocation you can have a look here: http://javahowto.blogspot.com/2006/06/6-common-errors-in-setting-java-heap.html