Increasing the size of rehearsal marks

[Alex Voice]

Hi Lilypond Experts,

I should like to increase the size of rehearsal marks relative to the stave.

I'm sure that it has something to do with  

\set Score.markformatter   and    #'font-size

but I can't work out how to put these two bits together and where.

I am sorry if there is a bit of the documentation I've missed, but I've tried looking in various places and even other websites!

Many thanks,

Alex Voice