I don’t want you to be another lost and confused foreigner when you come to Spain, so let me explain here and now: Spaniards don’t usually ask for a “cerveza.” They specify what size or type when ordering. Here are the most common options:
Una botella de cerveza
A standard glass bottle of beer, usually 12 ounces but sometimes smaller.
In general, the smallest size of draft beer a bar sells. Usually a small glass or wine glass with a little less than a half pint of beer.