\version "2.17"

\paper {
  oddHeaderMarkup = \markup \null
  oddFooterMarkup = \markup \null
}
#(set-global-staff-size
  (let ((gss (ly:parser-lookup parser 'mysize)))
    (if (number? gss) gss 14)))


allowGrobCallback =
#(define-scheme-function (parser location syms) (symbol-list?)
   (let ((interface (car syms))
         (sym (cadr syms)))
     #{
       \with {
         \consists #(lambda (context)
                      `((acknowledgers .
                          ((,interface . ,(lambda (engraver grob source-engraver)
                                            (let ((prop (ly:grob-property grob sym)))
                                              (if (procedure? prop) (ly:grob-set-property! grob sym (prop grob)))
                                              ))))
                          ))
                      )
       }
     #}))

absFontSize =
#(define-scheme-function (parser location pt)(number?)
   (lambda (grob)
     (let* ((layout (ly:grob-layout grob))
            (ref-size (ly:output-def-lookup (ly:grob-layout grob) 'text-font-size 12)))
       (magnification->font-size (/ pt ref-size))
       )))

\layout {
  \context {
    \Score
    \allowGrobCallback font-interface.font-size
    \override LyricText #'font-size = \absFontSize #42
    \override LyricText #'stencil =
    #(lambda (grob)
       (let* ((stil (lyric-text::print grob))
              (yx (ly:stencil-extent stil Y)))
         (ly:message "Y-extent-diff: ~A" (/ (cdr yx) (- (cdr yx)(car yx))))l
         stil))
  }
}

% just one letter ...  it should be 42pt
\new Lyrics \lyricmode { X }