With respect to using a coffee/spice grinder, I agree with Jim, based on my own personal experiences; 3/8inch works for me ( I just 'eyeball').

There are other ways of mixing longer fibers, one of which seems to be very practical, but which I have never tried. Put the material to be mixed into a large zip-lok bag and close it up. cut a small portion of one corner off, insert the small tube from a can of compressed air into the bag , and blast away! I have seen a YouTube video on how to do it , and it is impressive.