If you use what passes the 12mm and doesn't pass the 6mm, your bark will probably be on the large side, but that will depend on whether the largest % of the bark particles tend to be concentrated nearer the 1/2" or 1/4" size range. 1/8" - 1/4" is ideal.

Al

Al, here is the link for the screen I bought http://www.greenhousemegastore.com/product/garland-2-in1-sieve.

Pretty nifty - it would be the berries if they had a variety of mesh sizes available?
Al