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.