How small does it need to be? If you really need a full HTTP + servlets configuration then it might be easier to use one of the smaller Jetty configurations ( http://jetty.mortbay.org/ ).
Do you really need servlets (i.e. is this webapp meant to run anywhere)? Dropping the standard servlet interface will slim things down. Do you really need the ability to handle heavy traffic? Both Tomcat and Jetty put extra effort into handling large numbers of connections with high throughput - which translates to bigger code and data. If you don't need this ability, then a simpler HTTP server could be a better bet.
