It does get tiring to hear London hotel staff chuckle patronizingly and say, "You Americans are so used to large rooms" in a manner which suggests that WW2 hardships are still affecting the city. There are luxury hotels in London charging an arm and a leg that sell rooms which are 25 m2 in size -- I'm thinking of YOU, Four Seasons, and YOU, One Aldwych, just for starters. So you can't even go by quality level.

It's cheap to call the hotel and ask. If they don't appear to know the size of their rooms, assume the worst.