\version "2.17.25"

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

\score {
  <<
    \new Staff \with
       { instrumentName = "normal size" }
       { c'4 }
    \new Staff \with
       { instrumentName = "size -3"
         \staffSize #-3 }
       { c'4 }
    \new PianoStaff \with
       { instrumentName = "normal size" }
       <<
         \new Staff { c'4 }
         \new Staff { c'4 }
       >>
     \new PianoStaff \with
       { instrumentName = "size -5"
         \staffSize #-5 }
       <<
         \new Staff { c'4 }
         \new Staff { c'4 }
       >>
  >>
}