The size chart on this product says a small is for size 2 - 3, but it is actually for a size 6 months.  My daughter bought a large for her almost 2 year old twins.  Fortunately, they are on the skinny side, so they will be able to use them, but they are almost too small.  They wear 12 - 18 month pants.  The swim diapers themselves are better than the disposables, but I wish they would size them correctly.