Different size in different staffs

In a score for piano with other instruments it is common practice to print the two staffs for the piano at regular size and the other staffs for the other instruments at a smaller size above, forming a "two size system".

I have a sonata for violin and piano and would very much like to achieve this effect, but can't find a way to do it (version 1.1 on Mac). The advantage would be to make the music much clearer for the piano player because the smaller notes of the violin staff would never look like crowding the piano notes.