If a couple wants to get a room should they get a single or a double? It seems like a single bed is plenty of room for two people who are married. Or is it that if there are two people you have to get a double?

If you ask for a single bed, you are most likely going to get a twin bed, if you are lucky a full.
Make sure you say one room for a matrimonio . That implies a couple will be sharing a bed.

Well, in spanish you'd say you want a double room, or better, a room for two (una habitación para dos personas) , and they will probably ask you, or you'll tell them wether you want a "cama de matrimonio" (a double bed ) or "dos camas" (twin beds).