I love, love, love these slippers, and like most people, had to go up a size or two.  I am an 8 1/2 or 9 shoe size and had to order a 41 EU which is a 10 1/2.  Also, the size 40 was smaller than the size 39.  One foot was much tighter than the other.  I believe others have encountered this as well.  So, third time was the charm.  I'm glad I didn't give up and kept on trying as i really do love these slippers.