I used to use them bloody great Drennan Continentals in a size 4 back in the day, they were like anchors. 1/2lb silver bream used to pick them up. I don't think there is a carp alive that can't physically pick up a size 6.
My local commercial that i used to fish put loads of f1s in, a shy biting fish, i think not, rip roaring runs and 1lb f1s result on size 6s