My experience on cruises is that most cabins, including balcony cabins, are for two people, but there is often a sofa that can be made up into a bed for a third person. I have sailed P&O three times and this has been the case every time.
Beds are normally two single beds, but you can request them to be put together as a double - the size can vary, but mostly this makes a king size or larger. Sometimes you get a cover that disguises the joint between the two beds, but this is not always the case. It could be a bit uncomfortable for someone sleeping in the middle.