ATM that parameter is not easily configurable by you (you'd need to register your own instance of AutomaticWorkQueue into the bus for the application). The default value for the max number of threads processing @OneWay invocations is 256, meaning the server can serve up to 256 simultaneous @OneWay requests without blocking the jboss web thread that's serving the ws invocation (iow, allowing that to early return an HTTP 200 to the client).
If you think there's need for this configuration at JBossWS level, please create a jira for this.