This is a post directed to the Beginner Cook only (Son, are you reading?) Recipes commonly designate onion amounts as to size rather than weight or measured amounts. Just what is a "small" onion chopped anyway? Accoding to USDA, one large onion weighs 150 grams, or 7/8 cup chopped. One medium onion (2 1/2 inch diameter) weighs 110 grams, or 2/3 cup chopped. And one small onion weighs 70 grams, measuring 3/8 cup chopped. This provides a starting place for the apprentice, but of course onion is like walnuts, a little more (or less) is just fine according to individual tastes. Bare in mind that mincing onions and chopping will affect the volume measure too - though that's another post entirely for the beginner - another day!
