A double room could be two twin-sized beds in one room intended for two OR you may get a double bed - a bed big enough for two persons.
I have to tell you that in my experience beds in Spain are typically smaller than those in the USA in your standard hotel room but I think that's beginning to change with world-travel as it is today. I should also say that it seems to me the beds in hostels (called "hostales" in Spanish) are typically smaller/shorter-in-length than those in hotels
Good luck! Saludos, MadridMan
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