It is important to buy these shoes a size or two BIGGER than you normally wear.  If you buy them at your normal size they will fit but you will sweat a lot in them because there will be no ventilation and they will be less comfortable.  You should be able to fit about a finger breadth behind your heel.  I have worn them for YEARS like this and they work great.  I work as a surgeon and stand for hours in them and my feet never hurt whereas they used to hurt when I wore running shoes or dress shoes.  Buy them BIG.