This is a bit more tricky, since it's determined by the
space-function property, as described at
http://lilypond.org/doc/v2.2/Documentation/user/out-www/lilypond-internals/Beam.html
However, you are lucky! There's a good example in the Tips and Tricks
document on the web page, search for cue-notes.ly.
/Mats
I hope you don't find my answers too indirect, the intention is to help
you learn how to find the answer yourself the next time.
Jim Sabatke wrote:
I now have the thickness of the beams working right, but I have no
idea of how to make the space between them smaller.