In terms of classifying flake graphite I have found that it is regularly priced according to its US Mesh size. This is a link which provides a conversion table for assessing the reported flake size into US mesh - which for me makes it easier to compare prices - http://www.megagraphite.com/technical/meshtomicron. But basically it seems that the most desirable size is >297 microns, or 50 Mesh.
