How to set java heap size in Tomcat?

Stop Tomcat server, set environment variable CATALINA_OPTS, and then restart Tomcat. Look at the file tomcat-install/bin/catalina.sh or catalina.bat for how this variable is used.
 For example,

set CATALINA_OPTS=-Xms512m -Xmx512m  (Windows, no "" around the value)
export CATALINA_OPTS="-Xms512m -Xmx512m"  (ksh/bash, "" around the value)
setenv CATALINA_OPTS "-Xms512m -Xmx512m"  (tcsh/csh, "" around the value)

In catalina.bat or catallina.sh, you may have noticed CATALINA_OPTS, JAVA_OPTS, or both can be used to specify Tomcat JVM options. What is the difference between CATALINA_OPTS and JAVA_OPTS? The name CATALINA_OPTS is specific for Tomcat servlet container, whereas JAVA_OPTS may be used by other java applications (e.g., JBoss). Since environment variables are shared by all applications, we don't want Tomcat to inadvertently pick up the JVM options intended for other apps. I prefer to use CATALINA_OPTS.