Staff size - normal and Vaticana

[mpk]

Hi,
Could anybody help me to individually adjust the Vaticana staff-size, 
please?

\version "2.25.12"

#(set-global-staff-size 15)

\new VaticanaScore { 
  <<
    \new VaticanaVoice = "cantus" {\clef "vaticana-do3"
       a
       \[ g\melisma a g\melismaEnd  \]  f\ictus f
             \finalis
      
    }
    \new VaticanaLyrics \lyricsto "cantus" {
      Glo -- ri -- a in ex -- cel -- sis  De -- o.
    }
  >>
}

\layout {
  indent = 0
  ragged-last = ##t
  }

\new Score {
a b c' d'
}

Thanks, Martin