Bear in mind that Biblically you're to abide by the laws of the land in which you live.
Where I live, physical discipline would be considered abuse if:
- it were administered with anything other than an open hand (no belts, wooden spoons, hair brushes, or fists)
- the child is hit more than six times
- the child is hit hard enough for it to leave marks, welts or bruises.
Those seem to me to be good healthy guidelines, as a starting point?
OTOH (so to speak), my wife convinced me that my open hand was too hard and heavy, so I switched to a flip-flop. Not only is it nearly impossible to create an injury with a flip-flop, it's also less personal.
Someone else has mentioned, and I agree, that a child should never be spanked in anger or while angry. Any discipline should be delivered in sorrow, not in anger. Hold off until the anger subsides and the discipline is then onerous for you as well.
This doesn't mean not responding immediately. I remember when I was a kid and acted up in public: "You're going to get a whipping when we get home." It's the anticipation that gets you. And, yeah, boy, despite my pleading all the way home, she was good on that promise. That only happened once.
When I had to spank my five-year-old (with a flip-flop), he got the same treatment. "You know I will have to spank you for that when we get home." His plea: "But don't talk, okay?" I normally gave him about ten seconds of lecture between swats, so five swats took at least a minute.
Spanking has a very limited period of utility--I'd say a hand slap can be effective over the age of two, bending over the knee up to five. Before that age, they're too young to reason crime/punishment at all, above that age they reason well enough that physical punishment should not be necessary.
And some may not need it at all. I think my daughter got a light spanking once from my wife.
I did wind up giving my son a final spanking (pants down, over the knee with a flip-flop) when he was in the eighth grade. On that occasion, we discovered he had been "holding" a transister radio that his friend had stolen. The specific reason I pulled his trousers down and bent him over was to show him that if he got sent to "juvie," he was not to big for someone to pull his pants down and bend him over in "juvie." He understood that message, because we had discussed aberrant sex acts a couple of years earlier.
My children are now adults, btw, both married, and my son has three children of his own.