I agree that it is best to talk of stretching the hole or piercing as otherwise it does sound like it is the entire ear. In this case I think being less accurate will confuse people (everyone knows you have to plug things into an electrical socket, but not everyone knows much about enlarging piercings).
Fistula - not appropriate as this is a hole that results from an infection/absess, not something you do to yourself deliberately. It's a medical problem.
I wouldn't say 'piercing hole' either though as it's a bit repetitious. Just 'piercing' is the normal noun for this.