%%http://lsr.di.unimi.it/LSR/Item?id=862

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

theMusic = \relative c' { 
  a'8 a a a a \ff a a a
}

\score {
  <<
    \new Staff 
      { \theMusic }

    \new Staff \with {
      fontSize = #-3
      \override StaffSymbol.staff-space = #(magstep -3)
      \override StaffSymbol.thickness = #(magstep -3)
      } 
      { \theMusic }

    \new Staff 
    \with { \staffSize #-3 } 
      { \theMusic }
  >>
}