"\set staffSize" like "\set fontSize"?

[Felix Hammer]

Hello

I tried to do following. 
i have a system with two staffs. It should be a violin-duo.
the first staff should be bigger (staffsize) as the other staff an should have 
bigger notes (fontsize).
as you can see i set the fontsize with "\set fontSize = #+3". But i cannot set 
the staff-size the same way --> \set staff-size = #+3 

please can anybody tell me how to do this?

much thanks
felix

---------------------------------------------------------------------------------------------------

\score
{
  \notes <<
   \new Staff
   {
    \time 3/4
     \clef violin
      \tempo 4 =120
       \relative c''
        {
         \set fontSize = #+3 g4 c,8 d e f | g4 c, c | a' f8 g a b | c4 c, c | 
f g8 f e d | e4 f8 e d c | b4 c8 d e c | \grace e4( d2.)
         \break
          g4 c,8 d e f | g4 c, c | a' f8 g a b | c4 c, c | f g8 f e d | e4 f8 
e d c | d4 e8 d c b | c2. \bar ":|"
         \break
         e'4 c8 d e c | d4 g,8 a b g | c4 a8 b c g | fis4 e8 fis d4 | d8 e fis 
g a b | c4 b a | b d, fis | g2.
         \break
         g4 c,8 b c4 | a'4 c,8 b c4 | g' f e | d8 c b c d4 | g,8 a b c d e | 
f4 e d | e8 g c,4 b | c2. \bar ":|"
        }

    }
    \new Staff
    {
   [...]
    }
 >>
      %\midi { \tempo 4  = 120 }
}