Depending on the application of your cob, I would say you will be fine leaving modest-size stones in your cob. My
native clay has probably more sand and gravel (after rocks are removed) than clay and silt. For mass building, the aggregate may even be helpful, and certainly does not hurt. For finish layers, of
course, you want to sift out anything bigger than your intended thickness. For thermal cob in a
RMH bench, the more rocks the better - they will increase the density and thermal performance, while reducing the amount of clay you need to process.