Search:

I hope you are using jarsigner to sign the applet. If you need help with any parameter you can tell.
Signing JAR Files (The Java™ Tutorials > Deployment > Packaging Programs in JAR Files)

Java -cp $CLASSPATH:.:dom4j-1.6.1.jar:poi-ooxml-3.9-20121203.jarmlbeans-qname.jar:poi-ooxml-schemas-3.9-20121203.jar:poi-3.9-20121203.jar Size

Try compiling like this:
Javac -cp $CLASSPATH:.:dom4j-1.6.1.jar:poi-ooxml-3.9-20121203.jar:xmlbeans-qname.jar:poi-ooxml-schemas-3.9-20121203.jar:poi-3.9-20121203.jar Size.java

separate libraries...