I *was* able to put this in an included .ily file. I had been including the wrong file before
\version "2.19"
embiggenChordNames = #(define-scheme-function (size) (number?)
#{ \with {
chordNameFunction = #(lambda (in-pitches bass inversion context)
(define (helper mu)
(if (list? mu)
(if (eq? (car mu) musicglyph-markup)
(markup (#:fontsize (- size) mu))
(map helper mu))
mu))
(let ((orig (ignatzek-chord-names in-pitches bass inversion context)))
(markup (#:fontsize size (helper orig)))))
} #} )
Chords = \chordmode {
\override ChordName #'font-name = #"Source Sans Pro"
a:dim bes c:maj7 d:m e:aug fis g:7/b
}
<<
\new ChordNames \with \embiggenChordNames #3 { \Chords }
>>