How to set different bar number format every nth bar?

[Marc Mouries]

 I'd like to have bar numbers printed on every bar with the first bar 
number of each line bigger.
I'd like to find if there is any way to set different bar number format 
with a function rather than inline with the music.
I'd like to avoid to reset the bar number format for each staff like:

\score {
% set the size of the first bar number larger
  \override Score.BarNumber #'font-size = #-2
  c d e f | % 1st measure
  % 5th other measures on the same staff
  \override Score.BarNumber #'font-size = #-4
 g a b c | c d e f | g a b c | c d e f | g a b c |
  % and so on...
}

I've seen Neil's music function to set the visibility of the bar number 
every odd measure but don't think this can be reused to set the font size.

links to documentation i may have missed or scheme functions are welcome.