I don't believe I have ever stayed in a hotel in the US that had twin beds. I am certain that I have never stayed in a hotel in the US in which each room did not have its own bathroom.
In the US, if you asked for a "double room" the person may ask if you mean:
1) Two beds (which will be either doubles or queens)
2) A suite of two rooms
I expect a single, large bed would not be their first guess.
I'm pretty sure if you said to the average hotel clerk you wanted a room "en suite" he or she would ask you to clarify that you meant "a suite" (of two rooms).
(This applies to the major hotel chains. At the swanky hotels, they speak this language better.)
Ah, well this is definitely a US/UK difference then! Indeed, I have stayed in lots of hotels/motels in the USA, and have never seen a single bed. In fact, the beds in some of the rooms were about the same size as my flat in the UK!
Hotel rooms in general (if you're not talking about the posh, swanky ones in London), especially rooms in guesthouses, B&Bs and pubs that also have accommodation, are much smaller than they are in the USA. You pay extra in a lot of them for a room with its own bathroom. Many have a shared bathroom in the hall or perhaps your room will have its own bathroom, but it will still be accessed by leaving your room and going into the hall.
Single beds (3' wide and 6' 3" long) are much more common and, as I said, a twin room would usually comprise two single beds, separated by a small bedside table.
It's perhaps a little odd that in BrE, we seem to hang on to a French phrase (en-suite) for a private bathroom, though I guess it's quicker and takes up less space on a price list or website than "with private facilities" or "with private bathroom". The phrase is also used when describing the accommodation in a private house, for example when you are selling your house. It might be described as having "3 bedrooms, 1 en-suite" or perhaps "2 bathrooms, 1 en-suite".