Under control panel, go to Java or Java Plugin (names vary for all versions). Click on the Java Tab. Click on Applet Runtime Settings. Click on View Double-click on Java Runtime Settings box for the most recent version of Java (highest number). This box will be a small grey or white table cell.
For 128MBs of memory (double default, should be enough) enter: -Xmx128M For 256MBs of memory enter: -Xmx256M