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.