Setting entire document fonts

[Richard Shann]

Following the example for setting the entire document fonts at

http://www.lilypond.org/doc/v2.18/Documentation/notation/fonts#entire-document-fonts

I constructed this code:
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\version "2.18.0"
\paper  {
  #(define fonts
    (make-pango-font-tree "Nimbus Sans"
                          "Luxi Mono"
                          "Times New Roman"
                          (/ staff-height pt 20)))
}

{
         c'<> ^\markup {roman,
    \sans sans,
    \typewriter typewriter.} g' 
}
%#(set-global-staff-size 20)
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where I have switched round the order of the fonts so that the
typewriter should show in Roman and the Roman as sans etc.
This works, but if I uncomment the set-global-staff-size line it reverts
to the default.
What is going on here? How do I set the fonts and the overall staff
size? 

Richard