I wear a 9 all day, I got them in the 40, and they fit great. With the Velcro strap you can loosen it to wear with socks or wear it without socks and just tighten it a bit.  I like that they are warm without socks but don't make your feet sweat.  The cork foot bed provides a lot of support so they are easy to wear all day Sunday and my feet aren't killing me at the end of the day.  I already have a pair of Haflinger slippers for the past 6 years and they are still great. These will be a nice addition.