Una caña, por favor
Order a beer at a Spanish bar and the default is a caña, a small draught beer, usually around two hundred millilitres. The word means a reed or a cane, and refers to the tall, thin shape of the glass. A caña is not a half-pint in disguise. It is a deliberate size, built for the climate.
In a warm country a large glass of beer is warm before it is finished. A small one is drunk cold and replaced with a fresh one, and bar staff are quick to pour. The habit suits the Spanish way of drinking, which is a succession of short stops in different bars rather than a long evening in one.
What it is called where
The names change from region to region. In Madrid a taller glass is a tubo and a larger one a doble. In the Basque Country a small beer is a zurito. A clara is beer mixed with lemon soda, and is particularly popular in summer.
Visitors from northern Europe often find the portions comically small. Spaniards regard a litre of beer as a different kind of drink, for a different kind of day.