At this page, there are instructions for how to change chord symbol fonts:
My current score has chords like Bb/C. I can change the size of the letters and the flat-sign, but I can’t seem to change the size of the slash—so I’m wondering if there is a bug.
In the top menu, I choose Engrave / Font Styles…
then “Chord Symbols Altered Bass Separator Font” (isn’t that the slash, like in Bb/C?)
But it seems that changes made to this font do not affect the appearance of the slash in the score.