if you are planning to make a latte or cappuccino, i recommend as below formula.
Italian standard for latte or cappuccino is 6oz.
latte : 1oz espresso + 3.5 oz milk + 1.5 foam
cappuccino : 1oz espresso + 2.5 oz milk + 2.5 oz foam
1 oz is about 30 ml.
if your cup is 12 oz, you can just double the formula. however, this is just competition standard. if you personally like to add more espresso or more foam...etc, it is entirely up to you.
also, to answer your questions, you can use small one for your own cup and if you are making for two, probably bigger one.