I guess I am at about 400sqft minimum which is 40sqmt, although slightly smaller can work if the room is not jammed full of furniture. Even 400 sqft can fail if large wardrobes or strange massive pieces of furniture (think Andaz in NYC if you have ever been to any of them) fill the room and rob space.
I am seriously tired of room clerks and assistant managers who chuckle patronizingly at me and say, "You Americans are always wanting large rooms, but we here in Europe are all used to smaller rooms," sometimes still adding (I kid you not!), "because of the war."