I believe that should be in Java Opts
This is working for me.
It's at the top of my catalina.sh file (use catalina.bat for windows).
If you've installed
tomcat from the ".exe" installer, these settings are stored in the registry and can be configured from the Tocmat Configuration screen.
Start -> All Programs -> Apache Tomcat -> Tomcat Configuration -> Java (tab)
In any case, you can check the setting from the Tomcat Manager app.
Go to:
http://localhost:8080/manager/html Click on "Server Status" (right hand side of the page header)
Then look for the JVM section
Here's mine: