My 11month old son's foot measures 5 inches and I got the 18-24month size, which fit nicely. A little more room to grow but not that much.  He only wore Robeez with the super soft sole, so was a little freaked out by these sandals and cried and pointed at his feet lol after the shock wore off, he started to walk around and seems to like these shoes now.  He did fall a bit at first, but maybe it's because he's not used to the sole.