Changing the staff size

The simplest way to resize staves is to use

 #(set-global-staff-size size)

To resize an individual staff’s size, you can use the properties staff-space and fontSize.

<<
  \new Staff \relative c'' {
    \dynamicDown c8\ff c c c c c c c
  }
  \new Staff \with {
    fontSize = #-3
    \override StaffSymbol.staff-space = #(magstep -3)
  } \relative c {
    \clef bass c8 c c c c\f c c c
  }
>>