A medium-sized onion weighs approximately 0.5 pounds or 8 ounces. An alternative measuring solution is to use cups, with the amount of a chopped medium onion roughly equaling 1 cup.
There are many breeds and varieties of onions, such as white, red, sweet or yellow. The size doesn't differ so much from breed to breed. A small onion is the size of a lemon and weighs 0.25 pounds, a medium onion is the size of a navel orange, and a large one is the size of a small grapefruit weighing 1 pound. On a gastronomic level, a smaller onion is preferred to a larger one because it has a better flavor.