Reduced fontsize when using partcombine

[Trevor Daniels]

Hi

I've recently used partcombine to automatically produce a piano part to go
with an SATB choral work.  Very impressive!  But in a capella SATB the piano
part is for rehearsal only, and for these I want to reduce the size of the
printed notes in the piano reduction.  This is proving to be frustrating.
The simple
 \set fontSize=#-2
 reduces the size of notes in the two combined parts only when they are in
unison.
Adding
 \context Voice=one { \set fontSize=#-2 } and
 \context Voice=two { \set fontSize=#-2 },
reduces the notes when the two voices are separate but so far I've been
unable to reduce the notes when only one of the two voices is sounding (the
other having a rest).

Is this a bug? Is there a way to reduce _all_ the notes in a partcombined
staff in a simpler way, or if not how do I reduce the notes when just one of
the two voices is sounding?

I'm using LilyPond 2.8.6

Trevor