I've used 4 varieties. 9/0 and 10/0 stainless steel round body which were big, but still hooked stripers. They did tend to kill eels relatively quickly though, due to their large gauge.
I've used a 5/0 stainless steel round body which was a bit small gap wise for the gauge and which also worked, didn't kill eels, but was a bit tough to hook up.
Jon Rodin acquired some dark metal forged circle hooks which were about 5 or 6/0 and which were best of all.
FWIW - the sizing of circle hooks is a lot bigger than normal bait hooks - a 9/0 circle hook equated to about a 5/0 or 6/0 siwash hook. A 5/0 hook was about as big as a 2/0 hook.
L.