Does anyone know what range of fish an adult grey heron can take...?

Last year I lost a 5" golden rudd to the winged hunter, I also had a 9" koi
suffer injury. I am pleased to say that the koi responded to treatment, I
did feel a fool on my knees holding a fish in a wet towel spreading
antiseptic on its wounds.

Recently I have seen a few herons fly over but so far no visits. The fish
are pretty big now, mostly over 12" long.. even my orfe are this size.. I
can't believe the heron has not seem them so I wondered if the bird just
thought they were too big...