\version "2.19"

\markup\italic "Ossia staff's too long at key change:"

\new Staff = "main" \relative c'' {
  c4 b d c
  <<
    { c4 b d c }
    \new Staff \with {
      \omit TimeSignature
      alignAboveContext = #"main"
      \magnifyStaff #2/3
      firstClef = ##f
    }
    { e4 d f e }
  >>
  \key a\major
  c4 b c2
}

\markup\italic "First workaround (using StaffSymbol.width):"

\new Staff = "main" \relative c'' {
  c4 b d c
  <<
    { c4 b d c }
    \new Staff \with {
      \omit TimeSignature
      alignAboveContext = #"main"
      \magnifyStaff #2/3
      firstClef = ##f
      \override StaffSymbol.width = #41.75
    }
    { e4 d f e }
  >>
  \key a\major
  c4 b c2
}

\markup\italic "Second workaround (using KeySignature.stencil):"
\new Staff = "main" \relative c'' {
  c4 b d c
  <<
    { c4 b d c }
    \new Staff \with {
      \omit TimeSignature
      alignAboveContext = #"main"
      \magnifyStaff #2/3
      firstClef = ##f
      \override KeySignature.X-extent =  #'(0 . 0)
        \override KeySignature.stencil = #(lambda (grob) 
            (grob-interpret-markup grob 
              #{ 
                  \markup {
                    \with-color #white
                    \filled-box #'(-.75 . 3.3) #'(-1.5 . 1.5) #0 
                  }
              #}))
    }
    { 
      e4 d f e 
      \key a\major
    }
  >>
  \key a\major
  c4 b c2
}