Performance Staves = 7-9 mm
Choral/Vocal Staves = 6 mm
Conductor Staves = 4-9 mm
Study/Rehearsal Staves = 4-5 mm
Most software simply scales down normal staves to achieve smaller ones. When using small staves, if you can, thicken the lines and stems to improve legibility and prevent “dazzling”.
In piano scores, with performance staves and reference staves, always make the reference staves smaller so that they don’t compete with the music for the performer.
ND Music Edition 