A traditional cappuccino is 1 part espresso 1 part steamed milk 1 part foam.
At a standard 2 fl. oz of espresso, that would be the equivalent of 2 fl. oz. steamed milk and 2 fl. oz foam, requiring an 8 oz cup.
Note that a fl. oz (fluid ounce) is 30 mL (volume), not a unit of weight.
This is not what you get in most coffee bars.
Find other popular espresso drinks.
Wet or Dry
If you are ordering a cappuccino from one of the chain coffee shops, you can ask for it wet or dry. Wet means more milk and less foam; it is closer to a latte and is normally what they give you. Dry means less milk more foam, and is closer to the original recipe. I can tell if they got my dry cappuccino right when I pick it up.