In my opinion flakes would produce larger sheet as in powder, graphene sheet are like ann onion, when graphite during oxidation expands and exfoliates, sheet are not uniform in size. On the other hand, when flakes expand larger sheet will form but depending on oxidation-reduction process, they could wrap in theirselves, although this can avoided with proper suspension conditions, ion strenght to give an idea.

The smaller the graphite size, the easier to oxidize it, i.e., take less time, lower concentration of oxidant., in my opinion. We pllverized the graphite powders about 30 micrometre.

you need minimum Graphene flake size 2 micron to fabricate an electronic device.