From my experience I'd say that if you have something smaller than a size 9 foot and you're using 5mm socks, the 40-42 or whatever the euro #'s are for the Cressi 8-9, would be perfect. If you wear a size 9 and you use a 5mm sock though, go with the next size up. It's a real pain not being able to try on fins before you buy though.
Ron.
Ron.