My 13 month old daughter (18.5 lbs, 30" tall) wears a size 4 shoe. We have the Crocs Littles Clog in a size 2-3M
crocs Littles Clog (Infant/Little Kid), Ballerina Pink, 2-3 M US Infant
which fits my pre-walker perfectly with no room to grow. So I bought the Kids' crocs classic in a size 4 and although its a little bit bigger, she will outgrow it pretty soon. See the pics for size comparison. Other than that, the shoes are awesome, no blisters, seems comfortable and my baby is always excited to put them on.