optional cue notes

[Paul Scott]

How can I get two note heads on a single stem and have one of the note 
heads be smaller?

\version "2.23.5"
\score{
  \new Staff {
    \fixed c' {
      << { a }
     { \set Voice.fontSize = #-2 a' } >>
    }
  }
}

This makes note heads small.  I'm guessing this might be a scope question.

TIA for any help with this,

Paul