If the place wasn't busy, I would have asked politely if there was anything else available. It looks like you were deliberately given one of the least desirable rooms, possibly as "punishment" for paying the cheapest advance purchase rate. But as a practical matter, "room selected at check-in" to me means maybe you get a king, maybe two queens. I never quack about getting two queens as that's the bed size I sleep on at home anyway -- but I would try to improve from a single bed, especially if Diamond.