\version "2.19.8"
%=> http://lilypond.1069038.n5.nabble.com/With-markup-score-can-I-separately-control-the-score-size-td163772.html

\layout {
  indent = 0
}

theMusic = \relative f {
  c'8 d e f g a b c
}

\markup\left-column { "staffSize :" \vspace #1 }

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



\new Staff 
\with { \staffSize #-3 } 
{
  \mark\markup "#-3"
  \theMusic
}

\new Staff 
\with { \staffSize #0 } 
{
  \mark\markup "#0"
  \theMusic
}

\new Staff 
\with { \staffSize #3 } 
{
  \mark\markup "#3"
  \theMusic
}

\markup\left-column { "mySize :" \vspace #1 }
mySize =
#(define-scheme-function (parser location nmbr score) (number? ly:score?)
  #{ \markup \scale #(cons (+ 1 (* 0.1 nmbr)) (+ 1 (* 0.1 nmbr))) \score { #score } #})

\mySize #-3
\score {
  {
    \mark\markup "#-3"
    \theMusic
  }
}

\mySize #0
\score {
  {
    \mark\markup "#0"
    \theMusic
  }
}

\mySize #3
\score {
  {
    \mark\markup "#3"
    \theMusic
  }
}