Changing the staff size

Though the simplest way to resize staves is to use #(set-global-staff-size size), an individual staff's size can be changed by scaling the properties staff-space and fontSize.

\version "2.24"

<<
  \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)
  } {
    \clef bass
    c8 c c c c\f c c c
  }
>>