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

source
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 &quot;jumbo&quot; according to this data.