% Original example by Torsten Hämmerle
% http://lists.gnu.org/archive/html/lilypond-user/2018-06/msg00572.html

% \staffSize function from Cynthia Karl
% https://lists.gnu.org/archive/html/lilypond-user/2018-06/msg00663.html

\version "2.19.80"


staffSize = #(define-music-function (new-size) (number?)
#{
  \set fontSize = #new-size
  \override StaffSymbol #'staff-space = #(magstep new-size)
  \override StaffSymbol #'thickness = #(magstep new-size)
#})

#(set-default-paper-size "a6")

#(ly:message "test")

fluteI  = { \repeat unfold 20 { c''8 e'' g'' e'' } \bar "|." }
fluteII = { \repeat unfold 20 { c'4 g' } \bar "|." }

\bookpart {
  \header { instrument = "Full Score" }
  \score {
    \new StaffGroup \with { \staffSize #0 } <<
      \new Staff \with { instrumentName = "Fl. 1" \staffSize #-10 } \fluteI
      \new Staff \with { instrumentName = "Fl. 2" \staffSize #-10 } \fluteII
    >>
  }
}

\bookpart {
  \header { instrument = "Flute 1" }
  \score {
    \new Staff \fluteI
  }
}

\bookpart {
  \header { instrument = "Flute 2" }
  \score {
    \new Staff \fluteII
  }
}