Change size of all notes in one voice with partCombine

[Dinh Hoang Tu]

Hello Lilypond team and users,

I tried to combine 2 voices having identical rythm: notes of 2 voices
share same stem and same side, one note upper, other lower.
I found \partCombine is suitable for this.
Then I tried to make all notes of 2nd voice smaller, and found \tweak
can be used (I did tried \once, \single, \override, \overrideProperty
but none of them works as expected)
But the inconvenience is to tweak on every note of the 2nd voice, like below:

\version "2.22.1"
smallNote = \tweak font-size -3 \etc
soprano = { f'4 g'4 a'4 b'4 c''2 }
basso = { \smallNote d'4 \smallNote e' \smallNote f' \smallNote g'
\smallNote a'2 }
\new Staff << \partCombineUp \soprano \basso >>

Do we have a music function for this purpose? Or could someone help me
to create one?

Any help is appreciated.

Kind regards,
Tu' DINH