staff size and tremolo

[Libero Mureddu]

Hi!
First of all, congratulation for this nice and interesting software, 
that I'm trying to learn in order to free myself, in a near future, 
both from sibelius and finale!
That's my problem:
I want to reduce the size of one staff, in my case the piano staff, but 
the solution found in the docs -> tips and tricks -> staff-size.ly is 
applied after the \new Staff \with etc.
Organizing my piece I used the structure shown in templates -> 
jazz-combo.ly, where the \new exist in the \score only for the drums. I 
don't know where to put the lines to reduce the staff for the other 
parts. Here is the structure of my file:
 about the tremolo I receive the following warning:
warning: beam has less than two visible stems:
    \repeat "tremolo" 16 { g32 d'32
                                    } }
What's wrong wtih my tremolo?
I'm using lilypond 2.2.5
apple ibook g3 800
mac os 10.3.4

Thank you in advance for your help.

Libero Mureddu

Vanha Viertotie, 21 As. 415
00350 Helsinki - Finlandia
Tel. +358-9-5808415
Mob. +358-41-7702668
address@hidden

Via Abbiati, 4
20148 Milano - Italia
Tel. +39-02-4075953
Mob. +39-339-8757587

\version "2.2.5"

\header {
  title = "A Rameau"
  composer = "Libero Mureddu"
}
% ----- Piano -----
mdSuperiore = \notes {
\key g \major
 % \voiceOne
 \mark \default
 r4 r8 \relative g'' {g16 fis e d c b a}
  \relative g' {g fis e }\time 2/4
  | {d' c' b a g fis e fis}


}

mdInferiore = \notes {
 % \voiceTwo

}

msSuperiore = \notes {
  % \voiceOne
  }

msInferiore = \notes {
  \voiceTwo
  \key g \major
  \relative c {
    \repeat "tremolo" 16 { g32 d'32 } }
  }


PianoMD = {
  \clef treble
  \context Staff <<
    \context Voice = one \mdSuperiore
    \context Voice = two \mdInferiore
  >>
}

PianoMS = {
  \clef bass
  \context Staff <<
  \context Voice = One \msSuperiore
  \context Voice = Two \msInferiore
>>
}

piano = {
  \context PianoStaff <<
  \override PianoStaff.TimeSignature  #'style = #'numbered
  \set PianoStaff.instrument = "Pianoforte "
  \context Staff = upper \PianoMD
  \context Staff = lower\PianoMS
>>
}

% ----- Contrabbasso -----

basso = \notes {
  \key g\major
  | \relative c {g8. g'8 g,8 g'8. [g,8 g'] g,8~ [
  | g16 g'8 g,] g'8.
  | g,4 g8.
  }
}

basso = {
  \set Staff.instrument = "Contrabbasso"
  \override Staff.TimeSignature  #'style = #'numbered
  \clef bass
  \context Staff <<
    \basso
  >>
}

% ----- Batteria -----

su = \drums {
  hh4
}

giu = \drums {
  bd4
}
drumContents = {
  <<
  \override Staff.TimeSignature  #'style = #'numbered
    \set DrumStaff.instrument = "Batteria"
    \new DrumVoice { \voiceOne \su }
    \new DrumVoice { \voiceTwo \giu }
  >>
}

% ----- partitura -----
\score {
  <<
  \context PianoStaff = piano \piano
  \context Staff = basso \basso
  \new DrumStaff { \drumContents }
>>
}