Serhii's suggestion works and here is some more detail.
If you look in your installation's bin directory you will see catalina.sh or .bat scripts. If you look in these you will see that they run a setenv.sh or setenv.bat script respectively, if it exists, to set environment variables. The relevant environment variables are described in the comments at the top of catalina.sh/bat. To use them create, for example, a file $CATALINA_HOME/bin/setenv.sh with contents
export JAVA_OPTS="-server -Xmx512m"
For Windows you will need, in setenv.bat, something like
set JAVA_OPTS=-server -Xmx768m