How to get RCS of single particle?

It's shown that the RCS calculation of particles is based on the Muller Matrix, so the result of calculation should be RCS with unit of 'm^2/m^3', i.e., the RCS in a unit volume. So I'm wondering how to get the single particle's RCS with unit of 'm^2'.

I'm looking forward to your reply. Thank you for your kindness!