I have two pairs of W39s (TrekSport and Komodo LS) and they fit very snugly. They can be a little tight after a period of time, especially around the toes, but other than that, I don't feel like they are going anywhere. I tried a pair of W40s (Komodo LS), but I felt that my smallest two toes on each foot were no where near the toe pockets and I was also worried about the looser feel, so I returned them for one of the aforementioned W39s.
I now have a pair of M40s (Komodo LS, bought on sale) and these are looser than the w39s and if I push my toes into the front of the pockets, I can put my finger down the back of the shoe. However, my toes don't feel like they are going to fall out of the pockets, nor do they feel the same tightness as they do in the W39s, which actually makes them a lot more comfortable. With a pair of injini lightweight socks on, they don't feel particularly loose either.
I like these VFF shoes, but the randomness of the fit is a little frustrating. I may revisit some W40s, if the M40s are too loose and I feel like I need a tighter fit, but not as tight as the W39s. I will keep the W39s for gym use, where the snugness may be beneficial.
I would say W40, the M40 are somewhat wider, and a bit longer.
The being a small feet guy is something that could end up being solved. At www.dravesarchery.com and www.travelcountry.com they have some M38 and M39 in various styles <ASK through the messenger thing they have>. They are on sale, i think.
I'm reading, that for some, the socks influence sizing of their vibrams, for others they don't. But if you wear socks of different thickness, doesn't that matter regarding the fitting of your VFFs?
If the socks are thick <ex. wool> and your foot is on the big side of the size, they might make the shoe feel tight, but it depends on how the shoes feel/fit before the socks ans how thick the socks are. I do not use thick socks with shoes, only medium to thin ones, the thicks socks are reserved for very cold nights in sandals .