I wear an 8 or 8.5 so when ordering backless shoes I err on the side of caution and get a size larger so I don't end up with my heels smashing the back down.  These shoes seem to have taken that into account and are sized a bit more generously so the 9 I bought is actually too big and flops around on my feet unless I'm wearing huge thick fluffy insulated socks as padding.  If you want to wear them on their own or with a normal pair of socks, buy the size you would normally wear in any other shoe.  Buy the next size up and they'll be way to big.  I love them, they have a thick chunky sole so I can wear them outside of the house and they're really very comfortable and cozy.  This is my 2nd pair, I just wish Id remembered they run big and gotten my normal size.  Will definitely buy again (and again!)