How to increase heap space for Jetty?

[Ferdinand Soethe]

Aggregating my project with wholesite.pdf in dynamic mode I'm getting a 
error

HTTP ERROR: 500 Java+heap+space
which is probably due to limited heap space.
How/where can I add the 'xmx1024M' Parameter for Java to the Jetty 
configuration to increases heap space?

Thanks,
Ferdinand