Some rooms can be small (stayed in a few small ones over the years) and some more spacious (like the CP I satyed in last year, which was surprisingly roomy, 2 double beds, a sitting area, large desk, etc.) Once you have narrowed it down, I'd call and ask how roomy the rooms are - contrary to the above, I wouldn't expect them to know the square footage of the room - I've noticed in North America people know how large their rooms / homes are (me included) but I don't know any of my English friends who know how big their own houses are (3 bedroom / 4 bedroom is a usual description of size rather that square feet) so wouldn't expect them to know the hotel rooms that way. A general 'yes, there is plenty of space' or 'you might find it a bit tight' would be enough for me.

Also, given the nature of many London hotels (in older buildings), you may find one room you can't swing a cat, but another, exactly the same 'grade' room, you will have lots of room, as they are often built around existing architecture, rather than purpose built.