To determine the size of your desired pitcher, a good rule of thumb is as follows:

Determine how much milk you will need to steam for a drink (i.e. use 6-7 ounces for a latte in a 10 ounce cup). As a minimum, then double those ounces to determine the size of pitcher you will need.  For example, you need 6 ounces of milk. Then a 12 ounce pitcher would be needed. .