Changing staff size

[Jérôme Plût]

I saw in the documentation how to change the size of a single staff
(such as the solist part in a piano accompaniment score) with the
\with command. However, since I might need to do this for several
scores, I would like to factor out the \with snippet in a macro.

My question is therefore: is there a simple way to write a macro such
that
\new Staff \mymacro << ... >>
(or \new Staff << \mymacro ... >>)
would be equivalent to
\new Staff \with { [some properties set] }

 ? (I know about magnifyStaff in 2.19, but I only have 2.18 here).

Also two, maybe-related, questions:
 - what does the \with block do precisely? The index link for
   \with sends to
   
http://lilypond.org/doc/v2.18/Documentation/notation/modifying-context-plug_002dins#index-_005cwith-1
 ,
   which gives a few examples, but not the definition, of \with.

 - as I understand it, changing only fontSize and
   StaffSymbol.staff-space only reduces a single staff, but not the
   inter-staff symbol. I may access this via the StaffGrouper object,
   but what if the piano accompanies two instruments ? Say:
\new StaffGroup <<
  \new Staff \oboe
  \new Staff \clarinet
  \new PianoStaff << \new Staff \righthand \new Staff \lefthand >>
>>

Here, I would want to reduce the space between \oboe and \clarinet,
but not between \righthand and \lefthand.