This question always brings out interesting answers, so to add to the confusion:
Single and Double on the face of it applies only to the number of people staying in the room, nothing else. Rooms in Europe, at the places I stay, are far from standardized. Just specifying such will get you a variety of rooms and beds, depending on what the owner has. It would be entirely possible, but unlikely, that in requesting a single, you get put in a large room with 4 large beds...because that was the last room available. If you prefer a single "double" bed, the posters advice about requesting a "Matrimonial" bed is good, but could still be two small beds pushed together. While stickier if you have reservations, you can always ask to see the room before agreeing to stay (common practice among Europeans) and if it is not to your liking, ask to see something else or go elsewhere.