Staff size voice staff when combining with piano part?

I created a new layout on which I combined a voice and piano player. I always used a smaller sized staff for other players in a piano part, which I believe is common practice. What should be the staff size for the extra players in a piano part?

I think that’s really up to you.
I just checked in a Henle score for violin and piano (Mozart, violin sonatas): 2/3. Depending on how complex the music on the smaller staff is, it might be worth choosing a slightly larger size.

Thanks, so there is not really a hard rule for this. It’s more a practical choice of finding a good midway between readability and fitting on the page as a total.