If you downloaded and installed the ".exe" version of tomcat and you chose to install the "start menu items", you can configure the memory by clicking Start -> All Programs -> Apache Tomcat -> Configure Tomcat -> Java (tab), and filling out the "Intial Memory Pool" and "Maximum Memory Pool" fields.
If your app does have a memory leak, increasing the JVM heap size will only postpone the inevitable. Most memory leaks in a Java Web App are the result of connections not being returned to a connection pool. [ June 23, 2005: Message edited by: Ben Souther ]
Alternately, if you are executing startup.bat from a command prompt (which calls catalina.bat) you can set the JAVA_OPTS environment variable in the catalina.bat file with any of the Java command line options. (See the SDK tooldocs for the available options) Bill