problem with size

[fabio gabbianelli]

i change the size of one staff but it change the instrument name too....how can 
i do to change only the staff and not the instrument name? i use this: 
melody = .....
\set Staff.instrumentName = #"Violin"
.....
......
....
..
...
\score {
  <<\new Staff \with {
    
        fontSize = #-5
    
        \override StaffSymbol #'staff-space = #(magstep -5)

    }
    \new Voice = "mel" {\autoBeamOff \melody }
    \new PianoStaff <<
      \set PianoStaff.instrumentName = #"Piano"
      \new Staff = "upper" \upper
      \new Staff = "lower" \lower
    >>
  >>}

thanx!

ps: i write just the important things