It seems to me that 52" is the standard at least. I have no clue about efficiency and all that. That's probably a non-factor since the majority of fan blades are just flat. From my personal experience, it seems that a fan diameter between 1/3 and 1/2 the room width is the sweet spot for a proportionally appropriate fan size. I have a 52" i my smallest 10x12 bedroom and it suits the room nicely. I have a 60" in a room that's 11x11 and it's nice. All the so called "recommended fan size" charts seem to like fans that are goofily small. This is of course assuming you want a ceiling fan.
72" is about as large as it gets until you venture into the ridiculous prices.
72" is about as large as it gets until you venture into the ridiculous prices.
