The mixture can have small pieces of stems and roots without problems. If you plan to roll the herbs in cigarette papers, this becomes very important. Even small stems will poke holes in the paper. Remove all the stems. Powder or finely chop the slower burning roots and hard herbs.
Some herbs are especially helpful to obtain the proper physical consistency of the mixture.
By far the best physical base for a smoking mixture is Mullein. When prepared correctly, it is light and puffy. The other herbs mix well into it, and it will burn evenly when lit. It is a good carrier of the other substances. You can also use finely shredded barks.