In my experience that would vary a lot depending on the thickness of the cut. Once again not a form of measure that I would even consider. My Salter electronic scale is my preference for any portioning by weight.
I never really saw a golf ball sized onion (and I'm very familiar with golf balls, having played for 40 years). Even the smallest "standard" supermarket onion is between that and a tennis ball, but closer to the latter. I have bought onions that are close to softball size, but I usually try to pick what I call "medium", around tennis ball size. Once again, if quantity is critical, weight is the way to go. For things like onions, I'm as likely to just estimate and go.