How much is a small/medium/large onion?
Here's some general guidelines I have used in the past:
Small onion = 4 ounces by weight or about ½ cup chopped
Medium onion = 8 ounces, or about 1 cup chopped
Large onion = 12 ounces, or about 1½ cups chopped
Jumbo onion = 16 ounces, or about 2 cups chopped
The terms are not always completely accurate so use your best judgement. I've often seen large onions that were as big or bigger than "jumbo" according to this data.