Different font size settings for instrumentName and shortInstrumentName

Choose different font sizes for instrumentName and shortInstrumentName as a context override.

\version "2.24"

InstrumentNameFontSize =
#(define-music-function (font-size-pair)(pair?)
"Set the font size of `InstrumentName` grobs.
The first value of FONT-SIZE-PAIR sets the font size of the initial
`instrumentName` property, the second value sets the font size of
`shortInstrumentName`"

;; This code could be changed/extended to set different values for each
;; occurance of `shortInstrumentName'

#{
  \override InstrumentName.after-line-breaking = 
    #(lambda (grob)
       (let* ((orig (ly:grob-original grob))
              (siblings (if (ly:grob? orig)
                            (ly:spanner-broken-into orig)
                            '())))
         (if (pair? siblings)
             (begin
               (ly:grob-set-property! 
                 (car siblings) 
                 'font-size (car font-size-pair))
               (for-each
                 (lambda (g)
                   (ly:grob-set-property!
                   g
                   'font-size (cdr font-size-pair)))
                 (cdr siblings))))))
#})

\layout {
  indent = 3\cm
  short-indent = 0.8\cm

  \context {
    \Staff
    \InstrumentNameFontSize #'(6 . -3)
  }
} 

\new StaffGroup <<
  \new Staff \with {
       instrumentName = "Flute"
       shortInstrumentName = "Fl." } {
    c''1 \break c'' \break c'' }
  \new Staff \with {
       instrumentName = "Violin"
       shortInstrumentName = "Vl." } {
    c''1 \break c'' \break c'' } 
>>