\version "2.18.2"

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

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

\book {
  \header { instrument = "Flute 2 layout 12" }
  \score {
    \new Staff \fluteII
    \layout {
      #(layout-set-staff-size 12)
    }
  }
}

#(set-global-staff-size 17)
\bookpart {
  \header { instrument = "Full Score global 17" }
  \score {
    \new StaffGroup <<
      \new Staff \with { instrumentName = "Fl. 1" } \fluteI
      \new Staff \with { instrumentName = "Fl. 2" } \fluteII
    >>
  }
}

\bookpart {
  \header { instrument = "Flute 1 layout in mod 24" }
  \score {
    \new Staff \fluteI
    \layout {
      #(layout-set-absolute-staff-size-in-module
        (current-module) (* 24 (eval 'pt (current-module))))
    }
  }
}

\bookpart {
  \header { instrument = "Flute 2 layout 8" }
  \score {
    \new Staff \fluteII
    \layout {
      #(layout-set-staff-size 8)
    }
  }
}