The Crocs site has a downloadable and printable size chart. I'd use that. When we first got DS his Disney Caymans, he was too big for one size, but too small for the other. We had to wait to give them to him until his feet had grown a bit, and then he had to wear thick socks (thankfully we live in the pac NW where you're not shunned for wearing wool socks with sandals like that, LOL). Crocs are supposed to fit loosely all around.
That said, he was 3 when we got them for him. At 2 he was still mainly wearing the biggest Robeez and then See Kai Run, because the soles were nice and pliable. I waited for Crocs and even Stride Rite until he was really firm on his feet when walking and running, because I didn't want any thick soles tripping him up.
however, I just saw on the Crocs site that they are making little tennis shoe style crocs now, and that's just too cute!