From my own experience, searching the web, the comments I've received in my previous two posts(Vibram Five Fingers and Vibram Five Fingers: 1 Month Later) and in conversation, sizing the Vibram Five Fingers is quite a concern and challenge. This is understandable as they're not shoes in the sense we know it and they need to be snug. If they're too big, you will notice.
Well, here's my guide to sizing the Vibram Five Finger Classics based on my own experience. Your experience may vary.
I know this sounds odd, but this is what I've had to do. The 44's were way too big.
Hopefully this saves you a bit of time and removes the need to have to send your order back for a smaller pair.
Let me know how you get on with these instructions. All comments will help others who come to buy their own pair of Vibram Five Fingers.
Update: Begin to Dig has an indepth post on sizing the different Vibram models with and without the injiniji socks.