It depends, of course, on the specific scenario.
For most one-on-one situations, I prefer 2-4 inches. The ability to completely enclose her in my hand or mouth is an indispensable experience.
In a harem situation, however, I could definitely see a role for a “headmistress” who is up to 8 inches. Too big to swallow and bigger than the other girls, but still small enough to hold in one hand while tasting her.