How big is an onion?

I'm a bit obsessive compulsive about this, which has earned me much teasing at home.

I've noticed a huge range of size in the onions at the grocery store. They seem to range anywhere from the size of a tennis ball to the size of a softball. When a
recipe called for "one onion" I would have a hard time figuring out just how much that was supposed to be.

After a bit of experimenting, I've decided that "one onion" is 8oz of onion.

After I came up with that and put it into practice I was able to stop obsessing about it and get back to cooking.